THE  LIBRARY 

OF 

THE  UNIVERSITY 
OF  CALIFORNIA 

LOS  ANGELES 

GIFT  OF 

Dr.  Dinsnore  Alter 


CLIMATIC  CHANGES 

THEIR  NATURE  AND  CAUSES 


PUBLISHED  ON  THE  FOUNDATION 

ESTABLISHED  IN  MEMOEY  OF 

THEODOEE  L.  GLASGOW 


OTHER  BOOKS  BY  THE  SAME  AUTHORS 

ELLSWOETH  HUNTINGTON 

A.  Four  books  showing  the  development  of  knowledge  as  to  Historical  Pulsa- 

tions of  Climate. 
The  Pulse  of  Asia.  Boston,  1907. 

Explorations  in  Turkestan.  Expedition  of  1903.  Washington,  1905. 
Palestine  and  Its  Transformation.  Boston,  1911. 
The  Climatic  Factor,  as  Illustrated  in  Arid  America.  Washington,  1914. 

B.  Two  books  illustrating  the  effect  of  climate  on  man. 
Civilization  and  Climate.  New  Haven,  1915. 

World  Power  and  Evolution.  New  Haven,  1919. 

C.  Four  books  illustrating  the  general  principles  of  Geography. 
Asia:  A  Geography  Header.  Chicago,  1912. 

The  Red  Man's  Continent.  New  Haven,  1919. 

Principles  of  Human  Geography  (with  S.  W.  Gushing).  New  York,  1920. 

Business  Geography  (with  F.  E.  Williams).  New  York,  1922. 

D.  A  companion  to  the  present  volume. 

Earth  and  Sun:  An  Hypothesis  of  Weather  and  Sunspots.  New  Haven. 
In  press. 

STEPHEN  SARGENT  VISHER 

Geography,  Geology  and  Biology  of  Southern  Dakota.  Vermilion,  1912. 

The  Biology  of  Northwestern  South  Dakota.  Vermilion,  1914. 

The  Geography  of  South  Dakota.  Vermilion,  1918. 

Handbook  of  the  Geology  of  Indiana  (with  others).  Indianapolis,  1922. 

Hurricanes  of  Australia  and  the  South  Pacific.  Melbourne,  1922. 


CLIMATIC   CHANGES 

THEIR  NATURE  AND  CAUSES 

BY 
ELLSWORTH  HUNTINGTON 

Eeseareh  Associate  in  Geography  in  Yale  University 

AND 
STEPHEN  SARGENT  VISHER 

Associate  Professor  of  Geology 
in  Indiana  University 


NEW  HAVEN 

YALE  UNIVERSITY  PEESS 

LONDON:   HUMPHREY  MILFORD  :   OXFORD  UNIVERSITY  PRESS 
MDCCCCXXII 


COPYEIGHT  1922  BY 
YALE  UNIVERSITY  PEESS 


Published  1922. 


.0- 


THE  THEODORE  L.  GLASGOW  MEMORIAL 
PUBLICATION  FUND 

THE  present  volume  is  the  fifth  work  published  by  the  Yale 
University  Press  on  the  Theodore  L.  Glasgow  Memorial  Publica- 
tion Fund.  This  foundation  was  established  September  17,  1918, 
by  an  anonymous  gift  to  Yale  University  in  memory  of  Flight 
Sub-Lieutenant  Theodore  L.  Glasgow,  R.N.  He  was  born  in 
Montreal,  Canada,  and  was  educated  at  the  University  of  Toronto 
Schools  and  at  the  Royal  Military  College,  Kingston.  In  August, 
1916,  he  entered  the  Royal  Naval  Air  Service  and  in  July,  1917, 
went  to  France  with  the  Tenth  Squadron  attached  to  the  Twenty- 
second  Wing  of  the  Royal  Flying  Corps.  A  month  later,  August 
19,  1917,  he  was  killed  in  action  on  the  Ypres  front. 


879267 


TO 

THOMAS  CHROWDER  CHAMBERLIN 
OF  THE  UNIVERSITY  OF  CHICAGO 

WHOSE  CLEAR  AND  MASTERLY  DISCUSSION 

OP   THE  GREAT  PROBLEMS  OF   TERRESTRIAL  EVOLUTION 

HAS  BEEN  ONE  OF  THE  MOST  INSPIRING  FACTORS 

IN  THE  WRITING  OF  THIS  BOOK 


THERE  is  a  toy,  which  I  have  heard,  and  I  would  not  have 
it  given  over,  but  waited  upon  a  little.  They  say  it  is  ob- 
served in  the  Low  Countries  (I  know  not  in  what  part], 
that  every  five  and  thirty  years  the  same  kind  and  suit 
of  years  and  weathers  comes  about  again;  as  great  frosts, 
great  wet,  great  droughts,  warm  winters,  summers  with 
little  heat,  and  the.  like,  and  they  call  it  the  prime;  it  is  a 
thing  I  do  the  rather  mention,  because,  computing  back- 
wards, 1  have  found  some  concurrence. 

FKANCIS  BACON 


PREFACE 

UNITY  is  perhaps  the  keynote  of  modern  science. 
This  means  unity  in  time,  for  the  present  is  but 
the  outgrowth  of  the  past,  and  the  future  of  the 
present.  It  means  unity  of  process,  for  there  seems  to  be 
no  sharp  dividing  line  between  organic  and  inorganic, 
physical  and  mental,  mental  and  spiritual.  And  the  unity 
of  modern  science  means  also  a  growing  tendency  toward 
cooperation,  so  that  by  working  together  scientists  dis- 
cover much  that  would  else  have  remained  hid. 

This  book  illustrates  the  modern  trend  toward  unity  in 
all  of  these  ways.  First,  it  is  a  companion  volume  to 
Earth  and  Sun.  That  volume  is  a  discussion  of  the  causes 
of  weather,  but  a  consideration  of  the  weather  of  the 
present  almost  inevitably  leads  to  a  study  of  the  climate 
of  the  past.  Hence  the  two  books  were  written  originally 
as  one,  and  were  only  separated  from  considerations  of 
convenience.  Second,  the  unity  of  nature  is  so  great  that 
when  a  subject  such  as  climatic  changes  is  considered,  it 
is  almost  impossible  to  avoid  other  subjects,  such  as  the 
movements  of  the  earth's  crust.  Hence  this  book  not  only 
discusses  climatic  changes,  but  considers  the  causes  of 
earthquakes  and  attempts  to  show  how  climatic  changes 
may  be  related  to  great  geological  revolutions  in  the 
form,  location,  and  altitude  of  the  lands.  Thus  the  book 
has  a  direct  bearing  on  all  the  main  physical  factors 
which  have  molded  the  evolution  of  organic  life,  includ- 
ing man. 


xii  PREFACE 

In  the  third  place,  this  volume  illustrates  the  unity  of 
modern  science  because  it  is  preeminently  a  cooperative 
product.  Not  only  have  the  two  authors  shared  in  its 
production,  but  several  of  the  Yale  Faculty  have  also 
cooperated.  From  the  geological  standpoint,  Professor 
Charles  Schuchert  has  read  the  entire  manuscript  in  its 
final  form  as  well  as  parts  at  various  stages.  He  has 
helped  not  only  by  criticisms,  suggestions,  and  facts,  but 
by  paragraphs  ready  for  the  printer.  In  the  same  way 
in  the  domain  of  physics,  Professor  Leigh  Page  has  re- 
peatedly taken  time  to  assist,  and  either  in  writing  or  by 
word  of  mouth  has  contributed  many  pages.  In  astron- 
omy, the  same  cordial  cooperation  has  come  with  equal 
readiness  from  Professor  Frank  Schlesinger.  Professors 
Schuchert,  Schlesinger,  and  Page  have  contributed  so 
materially  that  they  are  almost  co-authors  of  the  volume. 
In  mathematics,  Professor  Ernest  W.  Brown  has  been 
similarly  helpful,  having  read  and  criticised  the  entire 
book.  In  certain  chemical  problems,  Professor  Harry  W. 
Foote  has  been  our  main  reliance.  The  advice  and  sugges- 
tions of  these  men  have  frequently  prevented  errors,  and 
have  again  and  again  started  new  and  profitable  lines  of 
thought.  If  we  have  made  mistakes,  it  has  been  because 
we  have  not  profited  sufficiently  by  their  cooperation.  If 
the  main  hypothesis  of  this  book  proves  sound,  it  is 
largely  because  it  has  been  built  up  in  constant  consulta- 
tion with  men  who  look  at  the  problem  from  different 
points  of  vision.  Our  appreciation  of  their  generous  and 
unstinted  cooperation  is  much  deeper  than  would  appear 
from  this  brief  paragraph. 

Outside  the  Yale  Faculty  we  have  received  equally 
cordial  assistance.  Professor  T.  C.  Chamberlin  of  the  Uni- 
versity of  Chicago,  to  whom,  with  his  permission,  we  take 
great  pleasure  in  dedicating  this  volume,  has  read  the 


PREFACE  xiii 

entire  proof  and  has  made  many  helpful  suggestions. 
We  cannot  speak  too  warmly  of  our  appreciation  not 
only  of  this,  but  of  the  way  his  work  has  served  for  years 
as  an  inspiration  in  the  preliminary  work  of  gathering 
data  for  this  volume.  Professor  Harlow  Shapley  of  Har- 
vard University  has  contributed  materially  to  the  chap- 
ter on  the  sun  and  its  journey  through  space ;  Professor 
Andrew  E.  Douglass  of  the  University  of  Arizona  has 
put  at  our  disposal  some  of  his  unpublished  results; 
Professors  S.  B.  Woodworth  and  Reginald  A.  Daly,  and 
Mr.  Robert  W.  Sayles  of  Harvard,  and  Professor  Henry 
F.  Reid  of  Johns  Hopkins  have  suggested  new  facts  and 
sources  of  information;  Professor  E.  R.  Cumings  of 
Indiana  University  has  critically  read  the  entire  proof; 
conversations  with  Professor  John  P.  Buwalda  of  the 
University  of  California  while  he  was  teaching  at  Yale 
make  him  another  real  contributor;  and  Mr.  Wayland 
Williams  has  contributed  the  interesting  quotation  from 
Bacon  on  page  x  of  this  book.  Miss  Edith  S.  Russell  has 
taken  great  pains  in  preparing  the  manuscript  and  in 
suggesting  many  changes  that  make  for  clearness.  Many 
others  have  also  helped,  but  it  is  impossible  to  make  due 
acknowledgment  because  such  contributions  have  become 
so  thoroughly  a  part  of  the  mental  background  of  the 
book  that  their  source  is  no  longer  distinct  in  the  minds 
of  the  authors. 

The  division  of  labor  between  the  two  authors  has  not 
followed  any  set  rules.  Both  have  had  a  hand  in  all  parts 
of  the  book.  The  main  draft  of  Chapters  VII,  VIII,  IX, 
XI,  and  XIII  was  written  by  the  junior  author ;  his  con- 
tributions are  also  especially  numerous  in  Chapters  X 
and  XV;  the  rest  of  the  book  was  written  originally  by 
the  senior  author. 


CONTENTS 

PAGE 

I.  The  Uniformity  of  Climate 1 

II.  The  Variability  of  Climate 16 

III.  Hypotheses  of  Climatic  Change    ....  33 

IV.  The  Solar  Cyclonic  Hypothesis     ....  51 
V.  The  Climate  of  History 64 

VI.  The  Climatic  Stress  of  the  Fourteenth  Cen- 
tury     98 

VII.  Glaciation  According  to  the  Solar  Cyclonic 

Hypothesis        110 

VIII.  Some  Problems  of  Glacial  Periods      ...  130 

IX.  The  Origin  of  Loess        .      .      .     .      .      .      .  155 

X.  Causes  of  Mild  Geological  Climates    .      .      .  166 
XL  Terrestrial  Causes  of  Climatic  Changes  .      .  188 
XII.  Post-Glacial    Crustal    Movements    and    Cli- 
matic Changes 215 

XIII.  The  Changing  Composition  of  Oceans  and 

Atmosphere 223 

XIV.  The  Effect  of  Other  Bodies  on  the  Sun    .      .  242 
XV.  The  Sun 's  Journey  through  Space      .      .      .  264 

XVI.  The  Earth's  Crust  and  the  Sun  285 


LIST  OF  ILLUSTRATIONS 

PAGE 

Fig.    1.  Climatic  changes  and  mountain  building          25 
Fig.    2.  Storminess     at     sunspot    maxima    vs. 

minima 54 

Fig.    3.  Relative  rainfall  at  times  of  increasing 

and  decreasing  sunspots      .      .      .      .     58, 59 
Fig.    4.  Changes  of  climate  in  California  and  in 

western  and  central  Asia     ....  75 
Fig.    5.  Changes  in  California  climate  for  2000 
years,  as  measured  by  growth  of  Se- 
quoia trees 77 

Fig.    6.  Distribution  of  Pleistocene  ice  sheets     .         123 
Fig.    7.  Permian  geography  and  glaciation    .     .         145 
Fig.    8.  Effect  of  diminution  of  storms  on  move- 
ment of  water 175 

Fig.    9.  Cretaceous  Paleogeography    ....         201 
Fig.  10.  Climatic  changes  of  140,000  years  as  in- 
ferred from  the  stars 279 

Fig.  11.  Sunspot  curve  showing  cycles,  1750  to 

1920 283 

Fig.  12.  Seasonal  distribution  of  earthquakes    .  299 

Fig.  13.  Wandering  of  the  pole  from  1890  to  1898        303 


TABLES 

PAGE 

1.  The  Geological  Time  Table 5 

2.  Types  of  Climatic  Sequence  .....  16 

3.  Correlation   Coefficients   between   Rainfall 

and  Growth  of  Sequoias  in  California    .  80 

4.  Correlation   Coefficients   between  Eainfall 

Eecords  in  California  and  Jerusalem      .  84 

5.  Theoretical    Probability    of    Stellar    Ap- 

proaches      260 

6.  Thirty-Eight  Stars  Having  Largest  Known 

Parallaxes 276,277 

7.  Destructive  Earthquakes  from  1800  to- 1899 

Compared  with  Sunspots 289 

8.  Seasonal  March  of  Earthquakes       .      .      .  295 

9.  Deflection  of  Path  of  Pole  Compared  with 

Earthquakes 305 

10.  Earthquakes  in  1903  to  1908  Compared  with 
Departures  of  the  Projected  Curve  of  the 
Earth's  Axis  from  the  Eulerian  Position  306 


CHAPTER  I 
THE  UNIFORMITY  OF  CLIMATE 

THE  role  of  climate  in  the  life  of  today  suggests  its 
importance  in  the  past  and  in  the  future.  No  hu- 
man being  can  escape  from  the  fact  that  his  food, 
clothing,  shelter,  recreation,  occupation,  health,  and 
energy  are  all  profoundly  influenced  by  his  climatic  sur- 
roundings. A  change  of  season  brings  in  its  train  some 
alteration  in  practically  every  phase  of  human  activity. 
Animals  are  influenced  by  climate  even  more  than  man, 
for  they  have  not  developed  artificial  means  of  protect- 
ing themselves.  Even  so  hardy  a  creature  as  the  dog 
becomes  notably  different  with  a  change  of  climate.  The 
thick-haired  "husky"  of  the  Eskimos  has  outwardly 
little  in  common  with  the  small  and  almost  hairless 
canines  that  grovel  under  foot  in  Mexico.  Plants  are  even 
more  sensitive  than  animals  and  men.  Scarcely  a  single 
species  can  flourish  permanently  in  regions  which  differ 
more  than  20 °C.  in  average  yearly  temperature,  and  for 
most  the  limit  of  successful  growth  is  100.1  So  far  as  we 
yet  know  every  living  species  of  plant  and  animal,  includ- 
ing man,  thrives  best  under  definite  and  limited  conditions 
of  temperature,  humidity,  and  sunshine,  and  of  the  com- 
position and  movement  of  the  atmosphere  or  water  in 
which  it  lives.  Any  departure  beyond  the  limits  means 
lessened  efficiency,  and  in  the  long  run  a  lower  rate  of 

i  W.  A.  Setchell :  The  Temperature  Interval  in  the  Geographical  Distribu- 
tion of  Marine  Algae;  Science,  Vol.  52,  1920,  p.  187. 


2  CLIMATIC  CHANGES 

reproduction  and  a  tendency  toward  changes  in  specific 
characteristics.  Any  great  departure  means  suffering  or 
death  for  the  individual  and  destruction  for  the  species. 

Since  climate  has  so  profound  an  influence  on  life 
today,  it  has  presumably  been  equally  potent  at  other 
times.  Therefore  few  scientific  questions  are  more  im- 
portant than  how  and  why  the  earth's  climate  has  varied 
in  the  past,  and  what  changes  it  is  likely  to  undergo  in 
the  future.  This  book  sets  forth  what  appear  to  be  the 
chief  reasons  for  climatic  variations  during  historic  and 
geologic  times.  It  assumes  that  causes  which  can  now  be 
observed  in  operation,  as  explained  in  a  companion 
volume  entitled  Earth  and  Sun,  and  in  such  books  as 
Humphreys'  Physics  of  the  Air,  should  be  carefully 
studied  before  less  obvious  causes  are  appealed  to.  It 
also  assumes  that  these  same  causes  will  continue  to 
operate,  and  are  the  basis  of  all  valid  predictions  as  to 
the  weather  or  climate  of  the  future. 

In  our  analysis  of  climatic  variations,  we  may  well 
begin  by  inquiring  how  the  earth's  climate  has  varied 
during  geological  history.  Such  an  inquiry  discloses  three 
great  tendencies,  which  to  the  superficial  view  seem  con- 
tradictory. All,  however,  have  a  similar  effect  in  provid- 
ing conditions  under  which  organic  evolution  is  able  to 
make  progress.  The  first  tendency  is  toward  uniformity, 
a  uniformity  so  pronounced  and  of  such  vast  duration 
as  to  stagger  the  imagination.  Superposed  upon  this 
there  seems  to  be  a  tendency  toward  complexity.  During 
the  greater  part  of  geological  history  the  earth's  climate 
appears  to  have  been  relatively  monotonous,  both  from 
place  to  place  and  from  season  to  season;  but  since  the 
Miocene  the  rule  has  been  diversity  and  complexity,  a 
condition  highly  favorable  to  organic  evolution.  Finally, 
the  uniformity  of  the  vast  eons  of  the  past  and  the 


THE  UNIFORMITY  OF  CLIMATE  3 

tendency  toward  complexity  are  broken  by  pulsatory 
changes,  first  in  one  direction  and  then  in  another.  To 
our  limited  human  vision  some  of  the  changes,  such  as 
glacial  periods,  seem  to  be  waves  of  enormous  propor- 
tions, but  compared  with  the  possibilities  of  the  universe 
they  are  merely  as  the  ripples  made  by  a  summer  zephyr. 
The  uniformity  of  the  earth's  climate  throughout  the 
vast  stretches  of  geological  time  can  best  be  realized  by 
comparing  the  range  of  temperature  on  the  earth  during 
that  period  with  the  possible  range  as  shown  in  the  entire 
solar  system.  As  may  be  seen  in  Table  1,  the  geological 
record  opens  with  the  Archeozoic  era,  or  "Age  of  Uni- 
cellular Life,"  as  it  is  sometimes  called,  for  the  preceding 
cosmic  time  has  left  no  record  that  can  yet  be  read. 
Practically  no  geologists  now  believe  that  the  beginning 
of  the  Archeozoic  was  less  than  one  hundred  million 
years  ago ;  and  since  the  discovery  of  the  peculiar  proper- 
ties of  radium  many  of  the  best  students  do  not  hesitate 
to  say  a  billion  or  a  billion  and  a  half.2  Even  in  the 
Archeozoic  the  rocks  testify  to  a  climate  seemingly  not 
greatly  different  from  that  of  the  average  of  geologic 
time.  The  earth's  surface  was  then  apparently  cool 
enough  so  that  it  was  covered  with  oceans  and  warm 
enough  so  that  the  water  teemed  with  microscopic  life. 
The  air  must  have  been  charged  with  water  vapor  and 
with  carbon  dioxide,  for  otherwise  there  seems  to  be  no 
possible  way  of  explaining  the  formation  of  mudstones 
and  sandstones,  limestones  of  vast  thickness,  carbona- 
ceous shales,  graphites,  and  iron  ores.3  Although  the 
Archeozoic  has  yielded  no  generally  admitted  fossils,  yet 
what  seem  to  be  massive  algae  and  sponges  have  been 

2  J.  Barrell :   Rhythms  and  the  Measurements  of  Geologic  Time ;   Bull. 
Geol.  Soe.  Am.,  Vol.  28,  Dec.,  1917,  pp.  745-904. 

sPirsson  and  Schuchert:  Textbook  of  Geology,  1915,  pp.  538-550. 


4  CLIMATIC  CHANGES 

found  in  Canada.  On  the  other  hand,  abundant  life  is 
believed  to  have  been  present  in  the  oceans,  for  by  no 
other  known  means  would  it  be  possible  to  take  from  the 
air  the  vast  quantities  of  carbon  that  now  form  carbona- 
ceous shales  and  graphite. 

In  the  next  geologic  era,  the  Proterozoic,  the  re- 
searches of  Walcott  have  shown  that  besides  the  marine 
algae  there  must  have  been  many  other  kinds  of  life.  The 
Proterozoic  fossils  thus  far  discovered  include  not  only 
microscopic  radiolarians  such  as  still  form  the  red  ooze 
of  the  deepest  ocean  floors,  but  the  much  more  signifi- 
cant tubes  of  annelids  or  worms.  The  presence  of  the 
annelids,  which  are  relatively  high  in  the  scale  of  organi- 
zation, is  generally  taken  to  mean  that  more  lowly  forms 
of  animals  such  as  coelenterates  and  probably  even  the 
mollusca  and  primitive  arthropods  must  already  have 
been  evolved.  That  there  were  many  kinds  of  marine 
invertebrates  living  in  the  later  Proterozoic  is  indicated 
by  the  highly  varied  life  and  more  especially  the  trilo- 
bites  found  in  the  oldest  Cambrian  strata  of  the  next 
succeeding  period.  In  fact  the  Cambrian  has  sponges, 
primitive  corals,  a  great  variety  of  brachiopods,  the 
beginnings  of  gastropods,  a  wonderful  array  of  trilobites, 
and  other  lowly  forms  of  arthropods.  Since,  under  the 
postulate  of  evolution,  the  life  of  that  time  forms  an  un- 
broken sequence  with  that  of  the  present,  and  since  many 
of  the  early  forms  differ  only  in  minor  details  from  those 
of  today,  we  infer  that  the  climate  then  was  not  very 
different  from  that  of  today.  The  same  line  of  reasoning 
leads  to  the  conclusion  that  even  in  the  middle  of  the 
Proterozoic,  when  multicellular  marine  animals  must 
already  have  been  common,  the  climate  of  the  earth  had 
already  for  an  enormous  period  been  such  that  all  the 
lower  types  of  oceanic  invertebrates  had  already  evolved. 


THE  UNIFORMITY  OF  CLIMATE 


TABLE  1 
THE  GEOLOGICAL  TIME  TABLE4 

COSMIC  TIME 

FORMATIVE  ERA.  Birth  and  growth  of  the  earth.  Beginnings  of 
the  atmosphere,  hydrosphere,  continental  platforms,  oceanic 
basins,  and  possibly  of  life.  No  known  geological  record. 

GEOLOGIC  TIME 

ARCHEOZOIC  ERA.  Origin  of  simplest  life. 
PROTEROZOIC  ERA.  Age  of  invertebrate  origins.  An  early  and  a  late 

ice  age,  with  one  or  more  additional  ones  indicated. 
PALEOZOIC  ERA.  Age  of  primitive  vertebrate  dominance. 

Cambrian  Period.  First  abundance  of  marine  animals  and  domi- 
nance of  trilobites. 

Ordovician  Period.  First  known  fresh-water  fishes. 
Silurian  Period.  First  known  land  plants. 
Devonian  Period.  First  known  amphibians.  "Table  Mountain" 

ice  age. 

Mississippian  Period.  Eise  of  marine  fishes  (sharks). 
Pennsylvanian  Period.  Rise  of  insects  and  first  period  of  marked 

coal  accumulation. 

Permian  Period.  Eise  of  reptiles.  Another  great  ice  age. 
MESOZOIC  ERA.  Age  of  reptile  dominance. 

Triassic  Period.  Eise  of  dinosaurs.  The  period  closes  with  a  cool 

climate. 

Jurassic  Period.  Eise  of  birds  and  flying  reptiles. 
Comartchean  Period.  Eise  of  flowering  plants  and  higher  insects. 
Cretaceous  Period.  Eise  of  archaic  or  primitive  mammalia. 
CENOZOIC  ERA.  Age  of  mammal  dominance. 

Early  Cenosoic  or  Eocene  and  Oligocene  time.  Eise  of  higher 
mammals.  Glaciers  in  early  Eocene  of  the  Laramide  Moun- 
tains. 
Late  Cenosoic  or  Miocene  and  Pliocene  time.  Transformation  of 

ape-like  animals  into  man. 
Glacial  or  Pleistocene  time.  Last  great  ice  age. 

PEESENT  TIME 

PSYCHOZOIC  ERA.  Age  of  man  or  age  of  reason.  Includes  the 
present  or  ' '  Eecent  time, ' '  estimated  to  be  probably  less  than 
30,000  years. 


*From  Charles  Schuchert  in  The  Evolution  of  the  Earth  and  Its  In- 
habitants: Edited  by  E.  S.  Lull,  New  Haven,  1918,  but  with  revisions  by 
Professor  Schuchert. 


6  CLIMATIC  CHANGES 

Moreover,  they  could  live  in  most  latitudes,  for  the  in- 
direct evidences  of  life  in  the  Archeozoic  and  Protero- 
zoic  rocks  are  widely  distributed.  Thus  it  appears  that 
at  an  almost  incredibly  early  period,  perhaps  many  hun- 
dred million  years  ago,  the  earth 's  climate  differed  only 
a  little  from  that  of  the  present. 

The  extreme  limits  of  temperature  beyond  which  the 
climate  of  geological  times  cannot  have  departed  can  be 
approximately  determined.  Today  the  warmest  parts  of 
the  ocean  have  an  average  temperature  of  about  30° C. 
on  the  surface.  Only  a  few  forms  of  life  live  where  the 
average  temperature  is  much  higher  than  this.  In  deserts, 
to  be  sure,  some  highly  organized  plants  and  animals  can 
for  a  short  time  endure  a  temperature  as  high  as  75 °C. 
(167°F.).  In  certain  hot  springs,  some  of  the  lowest  uni- 
cellular plant  forms  exist  in  water  which  is  only  a  little 
below  the  boiling  point.  More  complex  forms,  however, 
such  as  sponges,  worms,  and  all  the  higher  plants  and 
animals,  seem  to  be  unable  to  live  either  in  water  or  air 
where  the  temperature  averages  above  45 °C.  (113°F.) 
for  any  great  length  of  time  and  it  is  doubtful  whether 
they  can  thrive  permanently  even  at  that  temperature. 
The  obvious  unity  of  life  for  hundreds  of  millions  of 
years  and  its  presence  at  all  times  in  middle  latitudes  so 
far  as  we  can  tell  seem  to  indicate  that  since  the  be- 
ginning of  marine  life  the  temperature  of  the  oceans 
cannot  have  averaged  much  above  50 °C.  even  in  the 
warmest  portions.  This  is  putting  the  limit  too  high 
rather  than  too  low,  but  even  so  the  warmest  parts  of 
the  earth  can  scarcely  have  averaged  much  more  than 
20°  warmer  than  at  present. 

Turning  to  the  other  extreme,  we  may  inquire  how 
much  colder  than  now  the  earth 's  surface  may  have  been 
since  life  first  appeared.  Proterozoic  fossils  have  been 


THE  UNIFORMITY  OF  CLIMATE  7 

found  in  places  where  the  present  average  temperature 
approaches  0°C.  If  those  places  should  be  colder  than 
now  by  30 °C.,  or  more,  the  drop  in  temperature  at  the 
equator  would  almost  certainly  be  still  greater,  and  the 
seas  everywhere  would  be  permanently  frozen.  Thus 
life  would  be  impossible.  Since  the  contrasts  between_ 
summer  and  winter,  and  between  the  poles  and  the 
equator  seem  generally  to  have  been  less  in  the  past  than 
at  present,  the  range  through  which  the  mean  tempera- 
ture of  the  earth  as  a  whole  could  vary  without  utterly 
destroying  life  was  apparently  less  than  would  now  be 
the  case. 

These  considerations  make  it  fairly  certain  that  for  at 
least  several  hundred  million  years  the  average  tempera- 
ture of  the  earth's  surface  has  never  varied  more  than 
perhaps  30 °C.  above  or  below  the  present  level.  Even  this 
range  of  60°C.  (108°F.)  may  be  double  or  triple  the  range 
that  has  actually  occurred.  That  the  temperature  has  not 
passed  beyond  certain  narrow  limits,  whatever  their 
exact  degree,  is  clear  from  the  fact  that  if  it  had  done  so, 
all  the  higher  forms  of  life  would  have  been  destroyed. 
Certain  of  the  lowest  unicellular  forms  might  indeed  have 
persisted,  for  when  dormant  they  can  stand  great  ex- 
tremes of  dry  heat  and  of  cold  for  a  long  time.  Even 
so,  evolution  would  have  had  to  begin  almost  anew.  The 
supposition  that  such  a  thing  has  happened  is  untenable, 
for  there  is  no  hint  of  any  complete  break  in  the  record 
of  life  during  geological  times, — no  sudden  disappear- 
ance of  the  higher  organisms  followed  by  a  long  period 
with  no  signs  of  life  other  than  indirect  evidence  such  as 
occurs  in  the  Archeozoic. 

A  change  of  60°  C.  or  even  of  20°  in  the  average  tem- 
perature of  the  earth's  surface  may  seem  large  when 
viewed  from  the  limited  standpoint  of  terrestrial  ex- 


8  CLIMATIC  CHANGES 

perience.  Viewed,  however,  from  the  standpoint  of 
cosmic  evolution,  or  even  of  the  solar  system,  it  seems 
a  mere  trifle.  Consider  the  possibilities.  The  tempera- 
ture of  empty  space  is  the  absolute  zero,  or  —  273 °C. 
To  this  temperature  all  matter  must  fall,  provided  it 
exists  long  enough  and  is  not  appreciably  heated  by  colli- 
sions or  by  radiation.  At  the  other  extreme  lies  the 
temperature  of  the  stars.  As  stars  go,  our  sun  is  only 
moderately  hot,  but  the  temperature  of  its  surface  is 
calculated  to  be  nearly  7000° C.,  while  thousands  of  miles 
in  the  interior  it  may  rise  to  20,000°  or  100,000°  or  some 
other  equally  unknowable  and  incomprehensible  figure. 
Between  the  limits  of  the  absolute  zero  on  the  one  hand, 
and  the  interior  of  a  sun  or  star  on  the  other,  there  is 
almost  every  conceivable  possibility  of  temperature. 
Today  the  earth's  surface  averages  not  far  from  14° C., 
or  287°  above  the  absolute  zero.  Toward  the  interior, 
the  temperature  in  mines  and  deep  wells  rises  about  1°C. 
for  every  100  meters.  At  this  rate  it  would  be  over  500° C. 
at  a  depth  of  ten  miles,  and  over  5000°  at  100  miles. 

Let  us  confine  ourselves  to  surface  temperatures, 
which  are  all  that  concern  us  in  discussing  climate.  It 
has  been  calculated  by  Poynting5  that  if  a  small  sphere 
absorbed  and  re-radiated  all  the  heat  that  fell  upon  it, 
its  temperature  at  the  distance  of  Mercury  from  the  sun 
would  average  about  210° C.;  at  the  distance  of  Venus, 
85° ;  the  earth  27° ;  Mars  —30° ;  Neptune  —219°.  A  planet 
much  nearer  the  sun  than  is  Mercury  might  be  heated  to 
a  temperature  of  a  thousand,  or  even  several  thousand, 
degrees,  while  one  beyond  Neptune  would  remain  almost 
at  absolute  zero.  It  is  well  within  the  range  of  possibility 
that  the  temperature  of  a  planet's  surface  should  be 

5  J.  H.  Poynting:  Radiation  in  the  Solar  System:  Phil.  Trans.  A,  1903, 
202,  p.  525. 


THE  UNIFORMITY  OF  CLIMATE  9 

anywhere  from  near  — 273°C.  up  to  perhaps  5000°C.  or 
more,  although  the  probability  of  low  temperature  is 
much  greater  than  of  high.  Thus  throughout  the  whole 
vast  range  of  possibilities  extending  to  perhaps  10,000°, 
the  earth  claims  only  60°  at  most,  or  less  than  1  per  cent. 
This  may  be  remarkable,  but  what  is  far  more  remark- 
able is  that  the  earth's  range  of  60°  includes  what  seem 
to  be  the  two  most  critical  of  all  possible  temperatures, 
namely,  the  freezing  point  of  water,  0°C.,  and  the  tem- 
perature where  water  can  dissolve  an  amount  of  carbon 
dioxide  equal  to  its  own  volume.  The  most  remarkable 
fact  of  all  is  that  the  earth  has  preserved  its  temperature 
within  these  narrow  limits  for  a  hundred  million  years, 
or  perchance  a  thousand  million. 

To  appreciate  the  extraordinary  significance  of  this 
last  fact,  it  is  necessary  to  realize  how  extremely  critical 
are  the  temperatures  from  about  0°  to  40° C.,  and  how 
difficult  it  is  to  find  any  good  reason  for  a  relatively 
uniform  temperature  through  hundreds  of  millions  of 
years.  Since  the  dawn  of  geological  time  the  earth's 
temperature  has  apparently  always  included  the  range 
from  about  the  freezing  point  of  water  up  to  about  the 
point  where  protoplasm  begins  to  disintegrate.  Hender- 
son, in  The  Fitness  of  the  Environment,  rightly  says  that 
water  is  "the  most  familiar  and  the  most  important  of 
all  things."  In  many  respects  water  and  carbon  dioxide 
form  the  most  unique  pair  of  substances  in  the  whole 
realm  of  chemistry.  Water  has  a  greater  tendency  than 
any  other  known  substance  to  remain  within  certain 
narrowly  defined  limits  of  temperature.  Not  only  does  it 
have  a  high  specific  heat,  so  that  much  heat  is  needed  to 
raise  its  temperature,  but  on  freezing  it  gives  up  more 
heat  than  any  substance  except  ammonia,  while  none  of 
the  common  liquids  approach  it  in  the  amount  of  addi- 


10  CLIMATIC  CHANGES 

tional  heat  required  for  conversion  into  vapor  after  the 
temperature  of  vaporization  has  been  reached.  Again, 
water  substance,  as  the  physicists  call  all  forms  of  H^O, 
is  unique  in  that  it  not  only  contracts  on  melting,  but 
continues  to  contract  until  a  temperature  several  degrees 
above  its  melting  point  is  reached.  That  fact  has  a  vast 
importance  in  helping  to  keep  the  earth's  surface  at  a 
uniform  temperature.  If  water  were  like  most  liquids, 
the  bottoms  of  all  the  oceans  and  even  the  entire  body  of 
water  in  most  cases  would  be  permanently  frozen. 

Again,  as  a  solvent  there  is  literally  nothing  to  com- 
pare with  water.  As  Henderson6  puts  it:  " Nearly  the 
whole  science  of  chemistry  has  been  built  up  around 
water  and  aqueous  solution. ' '  One  of  the  most  significant 
evidences  of  this  is  the  variety  of  elements  whose  pres- 
ence can  be  detected  in  sea  water.  According  to  Hender- 
son they  include  hydrogen,  oxygen,  nitrogen,  carbon, 
chlorine,  sodium,  magnesium,  sulphur,  phosphorus,  which 
are  easily  detected;  and  also  arsenic,  caesium,  gold, 
lithium,  rubidium,  barium,  lead,  boron,  fluorine,  iron, 
iodine,  bromine,  potassium,  cobalt,  copper,  manganese, 
nickel,  silver,  silicon,  zinc,  aluminium,  calcium,  and 
strontium.  Yet  in  spite  of  its  marvelous  power  of  solu- 
tion, water  is  chemically  rather  inert  and  relatively 
stable.  It  dissolves  all  these  elements  and  thousands  of 
their  compounds,  but  still  remains  water  and  can  easily 
be  separated  and  purified.  Another  unique  property  of 
water  is  its  power  of  ionizing  dissolved  substances,  a 
property  which  makes  it  possible  to  produce  electric 
currents  in  batteries.  This  leads  to  an  almost  infinite 
array  of  electro-chemical  reactions  which  play  an  almost 
dominant  role  in  the  processes  of  life.  Finally,  no 
common  liquid  except  mercury  equals  water  in  its  power 

«L.  J.  Henderson:  The  Fitness  of  the  Environment,  1913. 


THE  UNIFORMITY  OF  CLIMATE  11 

of  capillarity.    This   fact   is   of   enormous   moment  in 
biology,  most  obviously  in  respect  to  the  soil. 

Although  carbon  dioxide  is  far  less  familiar  than 
water,  it  is  almost  as  important.  '  *  These  two  simple  sub- 
stances," says  Henderson,  "are  the  common  source  of 
every  one  of  the  complicated  substances  which  are  pro- 
duced by  living  beings,  and  they  are  the  common  end 
products  of  the  wearing  away  of  all  the  constituents  of 
protoplasm,  and  of  the  destruction  of  those  materials 
which  yield  energy  to  the  body. ' '  One  of  the  remarkable 
physical  properties  of  carbon  dioxide  is  its  degree  of 
solubility  in  water.  This  quality  varies  enormously  in 
different  substances.  For  example,  at  ordinary  pressures 
and  temperatures,  water  can  absorb  only  about  5  per 
cent  of  its  own  volume  of  oxygen,  while  it  can  take  up 
about  1300  times  its  own  volume  of  ammonia.  Now  for 
carbon  dioxide,  unlike  most  gases,  the  volume  that  can 
be  absorbed  by  water  is  nearly  the  same  as  the  volume 
of  the  water.  The  volumes  vary,  however,  according  to 
temperature,  being  absolutely  the  same  at  a  temperature 
of  about  15°C.  or  59°F.,  which  is  close  to  the  ideal  tem- 
perature for  man's  physical  health  and  practically  the 
same  as  the  mean  temperature  of  the  earth's  surface 
when  all  seasons  are  averaged  together.  "Hence,  when 
water  is  in  contact  with  air,  and  equilibrium  has  been 
established,  the  amount  of  free  carbonic  acid  in  a  given 
volume  of  water  is  almost  exactly  equal  to  the  amount 
in  the  adjacent  air.  Unlike  oxygen,  hydrogen,  and  nitro- 
gen, carbonic  acid  enters  water  freely ;  unlike  sulphurous 
oxide  and  ammonia,  it  escapes  freely  from  water.  Thus 
the  waters  can  never  wash  carbonic  acid  completely  out 
of  the  air,  nor  can  the  air  keep  it  from  the  waters.  It  is 
the  one  substance  which  thus,  in  considerable  quantities 
relative  to  its  total  amount,  everywhere  accompanies 


12  CLIMATIC  CHANGES 

water.  In  earth,  air,  fire,  and  water  alike  these  two  sub- 
stances are  always  associated. 

"Accordingly,  if  water  be  the  first  primary  con- 
stituent of  the  environment,  carbonic  acid  is  inevitably 
the  second, — because  of  its  solubility  possessing  an 
equal  mobility  with  water,  because  of  the  reservoir  of  the 
atmosphere  never  to  be  depleted  by  chemical  action  in 
the  oceans,  lakes,  and  streams.  In  truth,  so  close  is  the 
association  between  these  two  substances  that  it  is 
scarcely  correct  logically  to  separate  them  at  all;  to- 
gether they  make  up  the  real  environment  and  they  never 
part  company. ' " 

The  complementary  qualities  of  carbon  dioxide  and 
water  are  of  supreme  importance  because  these  two  are 
the  only  known  substances  which  are  able  to  form  a  vast 
series  of  complex  compounds  with  highly  varying  chemi- 
cal formulae.  No  other  known  compounds  can  give  off 
or  take  on  atoms  without  being  resolved  back  into  their 
elements.  No  others  can  thus  change  their  form  freely 
without  losing  their  identity.  This  power  of  change  with- 
out destruction  is  the  fundamental  chemical  character- 
istic of  life,  for  life  demands  complexity,  change,  and 
growth. 

In  order  that  water  and  carbon  dioxide  may  combine 
to  form  the  compounds  on  which  life  is  based,  the  water 
must  be  in  the  liquid  form,  it  must  be  able  to  dissolve 
carbon  dioxide  freely,  and  the  temperature  must  not  be 
high  enough  to  break  up  the  highly  complex  and  delicate 
compounds  as  soon  as  they  are  formed.  In  other  words, 
the  temperature  must  be  above  freezing,  while  it  must 
not  rise  higher  than  some  rather  indefinite  point  between 
50° C.  and  the  boiling  point,  where  all  water  finally  turns 
into  vapor.  In  the  whole  range  of  temperature,  so  far  as 

7  Henderson :  loc.  cit.,  p.  138. 


THE  UNIFORMITY  OF  CLIMATE 


we  know,  there  is  no  other  interval  where  any  such  com- 
plex reactions  take  place.  The  temperature  of  the  earth 
for  hundreds  of  millions  of  years  has  remained  firmly 
fixed  within  these  limits. 

The  astonishing  quality  of  the  earth's  uniformity  of 
temperature  becomes  still  more  apparent  when  we  con- 
sider the  origin  of  the  sun's  heat.  What  that  origin  is 
still  remains  a  question  of  dispute.  The  old  ideas  of  a 
burning  sun,  or  of  one  that  is  simply  losing  an  original 
supply  of  heat  derived  from  some  accident,  such  as  colli- 
sion with  another  body,  were  long  ago  abandoned.  The 
impact  of  a  constant  supply  of  meteors  affords  an  almost 
equally  unsatisfactory  explanation.  Moulton8  states  that 
if  the  sun  were  struck  by  enough  meteorites  to  keep  up 
its  heat,  the  earth  would  almost  certainly  be  struck  by 
enough  so  that  it  would  receive  about  half  of  1  per  cent 
as  much  heat  from  them  as  from  the  sun.  This  is  millions 
of  times  more  he.at  than  is  now  received  from  meteors. 
If  the  sun  owes  its  heat  to  the  impact  of  larger  bodies  at 
longer  intervals,  the  geological  record  should  show  a 
series  of  interruptions  far  more  drastic  than  is  actually 
the  case. 

It  has  also  been  supposed  that  the  sun  owes  its  heat 
to  contraction.  If  a  gaseous  body  contracts  it  becomes 
warmer.  Finally,  however,  it  must  become  so  dense  that 
its  rate  of  contraction  diminishes  and  the  process  ceases. 
Under  the  sun's  present  condition  of  size  and  density  a 
radial  contraction  of  120  feet  per  year  would  be  enough 
to  supply  all  the  energy  now  radiated  by  that  body.  This 
seems  like  a  hopeful  source  of  energy,  but  Kelvin  cal- 
culated that  twenty  million  years  ago  it  was  ineffective 
and  ten  million  years  hence  it  will  be  equally  so.  More- 
over, if  this  is  the  source  of  heat,  the  amount  of  radia- 

sF.  E.  Moulton:  Introduction  to  Astronomy,  1916. 


14  CLIMATIC  CHANGES 

tion  from  the  sun  would  have  to  vary  enormously. 
Twenty  million  years  ago  the  sun  would  have  extended 
nearly  to  the  earth's  orbit  and  would  have  been  so  tenu- 
ous that  it  would  have  emitted  no  more  heat  than  some 
of  the  nebulae  in  space.  Some  millions  of  years  later, 
when  the  sun's  radius  was  twice  as  great  as  at  present, 
that  body  would  have  emitted  only  one-fourth  as  much 
heat  as  now,  which  would  mean  that  on  the  earth's  sur- 
face the  theoretical  temperature  would  have  been  200° 
below  the  present  level.  This  is  utterly  out  of  accord  with 
the  uniformity  of  climate  shown  by  the  geological  record. 
In  the  future,  if  the  sun's  contraction  is  the  only  source 
of  heat,  the  sun  can  supply  the  present  amount  for  only 
ten  million  years,  which  would  mean  a  change  utterly 
unlike  anything  of  which  the  geological  record  holds 
even  the  faintest  hint.9 

Altogether  the  problem  of  how  the  sun  can  have  re- 
mained so  uniform  and  how  the  earth's  atmosphere  and 
other  conditions  can  also  have  remained  so  uniform 
throughout  hundreds  of  millions  of  years  is  one  of  the 
most  puzzling  in  the  whole  realm  of  nature.  If  appeal  is 
taken  to  radioactivity  and  the  breaking  up  of  uranium 
into  radium  and  helium,  conditions  can  be  postulated 
which  will  give  the  required  amount  of  energy.  Such  is 
also  the  case  if  it  be  supposed  that  there  is  some  unknown 
process  which  may  induce  an  atomic  change  like  radio- 
activity in  bodies  which  are  now  supposed  to  be  stable 
elements.  In  either  case,  however,  there  is  as  yet  no 
satisfactory  explanation  of  the  uniformity  of  the  earth's 
climate.  A  hundred  million  or  a  thousand  million  years 
ago  the  temperature  of  the  earth's  surface  was  very 
much  the  same  as  now.  The  earth  had  then  presumably 
ceased  to  emit  any  great  amount  of  heat,  if  we  may  judge 

9  Moulton :  loc.  cit. 


THE  UNIFORMITY  OF  CLIMATE  15 

from  the  fact  that  its  surface  was  cool  enough  so  that 
great  ice  sheets  could  accumulate  on  low  lands  within  40° 
of  the  equator.  The  atmosphere  was  apparently  almost 
like  that  of  today,  and  was  almost  certainly  not  different 
enough  to  make  up  for  any  great  divergence  of  the  sun 
from  its  present  condition.  We  cannot  escape  the  stu- 
pendous fact  that  in  those  remote  times  the  sun  must 
have  been  essentially  the  same  as  now,  or  else  that  some 
utterly  unknown  factor  is  at  work. 


CHAPTER  II 


THE  VARIABILITY  OF  CLIMATE 

THE  variability  of  the  earth's  climate  is  almost  as 
extraordinary  as  its  uniformity.  This  variability 
is  made  up  partly  of  a  long,  slow  tendency  in  one 
direction  and  partly  of  innumerable  cycles  of  every  con- 
ceivable duration  from  days,  or  even  hours,  up  to  millions 
of  years.  Perhaps  the  easiest  way  to  grasp  the  full  com- 
plexity of  the  matter  is  to  put  the  chief  types  of  climatic 
sequence  in  the  form  of  a  table. 


TABLE  2 
TYPES  OF  CLIMATIC  SEQUENCE 


Cosmic  uniformity. 
Secular  progression. 
Geologic  oscillations. 
Glacial  fluctuations. 
Orbital  precessions. 
6.  Historical  pulsations. 


7.  Bruckner  periods. 

8.  Sunspot  cycles. 

9.  Seasonal  alternations. 

10.  Pleionian  migrations. 

11.  Cyclonic  vacillations. 

12.  Daily  vibrations. 


In  assigning  names  to  the  various  types  an  attempt 
has  been  made  to  indicate  something  of  the  nature  of  the 
sequence  so  far  as  duration,  periodicity,  and  general 
tendencies  are  concerned.  Not  even  the  rich  English 
language  of  the  twentieth  century,  however,  furnishes 
words  with  enough  shades  of  meaning  to  express  all  that 


THE  VARIABILITY  OF  CLIMATE  17 

is  desired.  Moreover,  except  in  degree,  there  is  no  sharp 
distinction  between  some  of  the  related  types,  such  as 
glacial  fluctuations  and  historic  pulsations.  Yet,  taken  as 
a  whole,  the  table  brings  out  the  great  contrast  between 
two  absolutely  diverse  extremes.  At  the  one  end  lies  well- 
nigh  eternal  uniformity,  or  an  extremely  slow  progress  in 
one  direction  throughout  countless  ages;  at  the  other, 
rapid  and  regular  vibrations  from  day  to  day,  or  else 
irregular  and  seemingly  unsystematic  vacillations  due  to 
cyclonic  storms,  both  of  which  types  are  repeated  mil- 
lions of  times  during  even  a  single  glacial  fluctuation. 

The  meaning  of  cosmic  uniformity  has  been  explained 
in  the  preceding  chapter.  Its  relation  to  the  other  types 
of  climatic  sequences  seems  to  be  that  it  sets  sharply 
defined  limits  beyond  which  no  changes  of  any  kind  have 
ever  gone  since  life,  as  we  know  it,  first  began.  Secular 
progression,  on  the  other  hand,  means  that  in  spite  of  all 
manner  of  variations,  now  this  way  and  then  the  other, 
the  normal  climate  of  the  earth,  if  there  is  such  a  thing, 
has  on  the  whole  probably  changed  a  little,  perhaps  be- 
coming more  complex.  After  each  period  of  continental 
uplift  and  glaciation — for  such  are  preeminently  the 
times  of  complexity — it  is  doubtful  whether  the  earth  has 
ever  returned  to  quite  its  former  degree  of  monotony. 
Today  the  earth  has  swung  away  from  the  great  diversity 
of  the  glacial  period.  Yet  we  still  have  contrasts  of  what 
seem  to  us  great  magnitude.  In  low  depressions,  such  as 
Turfan  in  the  central  deserts  of  Eurasia,  the  thermom- 
eter sometimes  ranges  from  0°F.  in  the  morning  to  60° 
in  the  shade  at  noon.  On  a  cloudy  day  in  the  Amazon 
forest  close  to  the  seashore,  on  the  contrary,  the  tempera- 
ture for  months  may  rise  to  85°  by  day  and  sink  no  lower 
than  75°  at  night. 

The  reasons  for  the  secular  progression  of  the  earth's 


18  CLIMATIC  CHANGES 

climate  appear  to  be  intimately  connected  with  those 
which  have  caused  the  next,  and,  in  many  respects,  more 
important  type  of  climatic  sequence,  which  consists  of 
geological  oscillations.  Both  the  progression  and  the 
oscillations  seem  to  depend  largely  on  three  purely  ter- 
restrial factors:  first,  the  condition  of  the  earth's  in- 
terior, including  both  internal  heat  and  contraction; 
second,  the  salinity  and  movement  of  the  ocean;  and 
third,  the  composition  and  amount  of  the  atmosphere. 
To  begin  with  the  earth's  interior — its  loss  of  heat  ap- 
pears to  be  an  almost  negligible  factor  in  explaining 
either  secular  progression  or  geologic  oscillation.  Accord- 
ing to  both  the  nebular  and  the  planetesimal  hypotheses, 
the  earth's  crust  appears  to  be  colder  now  than  it  was 
hundreds  or  thousands  of  millions  of  years  ago.  The 
emission  of  internal  heat,  however,  had  probably  ceased 
to  be  of  much  climatic  significance  near  the  beginning  of 
the  geological  record,  for  in  southern  Canada  glaciation 
occurred  very  early  in  the  Proterozoic  era.  On  the  other 
hand,  the  contraction  of  the  earth  has  produced  remark- 
able effects  throughout  the  whole  of  geological  time.  It 
has  lessened  the  earth's  circumference  by  a  thousand 
miles  or  more,  as  appears  from  the  way  in  which  the 
rocks  have  been  folded  and  thrust  bodily  over  one 
another.  According  to  the  laws  of  dynamics  this  must 
have  increased  the  speed  of  the  earth's  rotation,  thus 
shortening  the  day,  and  also  having  the  more  important 
effect  of  increasing  the  bulge  at  the  equator.  On  the  other 
hand,  recent  investigations  indicate  that  tidal  retardation 
has  probably  diminished  the  earth's  rate  of  rotation 
more  than  seemed  probable  a  few  years  ago,  thus  length- 
ening the  day  and  diminishing  the  bulge  at  the  equator. 
Thus  two  opposing  forces  have  been  at  work,  one  caus- 
ing acceleration  and  one  retardation.  Their  combined 


THE  VARIABILITY  OF  CLIMATE  19 

effect  may  have  been  a  factor  in  causing  secular  progres- 
sion of  climate.  It  almost  certainly  was  of  much  im- 
portance in  causing  pronounced  oscillations  first  one  way  > 
and  then  the  other.  This  matter,  together  with  most  of 
those  touched  in  these  first  chapters,  will  be  expanded  in 
later  parts  of  the  book.  On  the  whole  the  tendency  ap- 
pears to  have  been  to  create  climatic  diversity  in  place 
of  uniformity. 

The  increasing  salinity  of  the  oceans  may  have  been 
another  factor  in  producing  secular  progression,  al- 
though of  slight  importance  in  respect  to  oscillations. 
While  the  oceans  were  still  growing  in  volume,  it  is  gen- 
erally assumed  that  they  must  have  been  almost  fresh 
for  a  vast  period,  although  Chamberlin  thinks  that  the 
change  in  salinity  has  been  much  less  than  is  usually 
supposed.  So  far  as  the  early  oceans  were  fresher  than 
those  of  today,  their  deep-sea  circulation  must  have  been 
less  hampered  than  now  by  the  heavy  saline  water  which 
is  produced  by  evaporation  in  warm  regions.  Although^ 
this  saline  water  is  warm,  its  weight  causes  it  to  descend,lj 
instead  of  moving  poleward  in  a  surface  current;  this 
descent  slows  up  the  rise  of  the  cold  water  which  has 
moved  along  in  the  depths  of  the  ocean  from  high  lati- 
tudes, and  thus  checks  the  general  oceanic  circulation. 
If  the  ancient  oceans  were  fresher  and  hence  had  a  freer 
circulation  than  now,  a  more  rapid  interchange  of  polar 
and  equatorial  water  presumably  tended  to  equalize  the 
climate  of  all  latitudes. 

Again,  although  the  earth's  atmosphere  has  probably 
changed  far  less  during  geological  times  than  was 
formerly  supposed,  its  composition  has  doubtless  varied. 
The  total  volume  of  nitrogen  has  probably  increased,  for 
that  gas  is  so  inert  that  when  it  once  becomes  a  part  of 
the  air  it  is  almost  sure  to  stay  there.  On  the  other  hand, 


20  CLIMATIC  CHANGES 

the  proportions  of  oxygen,  carbon  dioxide,  and  water 
vapor  must  have  fluctuated.  Oxygen  is  taken  out  con- 
stantly by  animals  and  by  all  the  processes  of  rock 
weathering,  but  on  the  other  hand  the  supply  is  increased 
when  plants  break  up  new  carbon  dioxide  derived  from 
volcanoes.  As  for  the  carbon  dioxide,  it  appears  prob- 
able that  in  spite  of  the  increased  supply  furnished  by 
volcanoes  the  great  amounts  of  carbon  which  have  gradu- 
ally been  locked  up  in  coal  and  limestone  have  appre- 
ciably depleted  the  atmosphere.  Water  vapor  also  may 
be  less  abundant  now  than  in  the  past,  for  the  presence 
of  carbon  dioxide  raises  the  temperature  a  little  and 
thereby  enables  the  air  to  hold  more  moisture.  When  the 
area  of  the  oceans  has  diminished,  and  this  has  recurred 
very  often,  this  likewise  would  tend  to  reduce  the  water 
vapor.  Moreover,  even  a  very  slight  diminution  in  the 
amount  of  heat  given  off  by  the  earth,  or  a  decrease  in 
evaporation  because  of  higher  salinity  in  the  oceans, 
would  tend  in  the  same  direction.  Now  carbon  dioxide  and 
water  vapor  both  have  a  strong  blanketing  effect  whereby 
heat  is  prevented  from  leaving  the  earth.  Therefore,  the 
probable  reduction  in  the  carbon  dioxide  and  water 
vapor  of  the  earth's  atmosphere  has  apparently  tended 
to  reduce  the  climatic  monotony  and  create  diversity  and 
complexity.  Hence,  in  spite  of  many  reversals,  the  gen- 
eral tendency  of  changes,  not  only  in  the  earth's  interior 
and  in  the  oceans,  but  also  in  the  atmosphere,  appears  to 
be  a  secular  progression  from  a  relatively  monotonous 
climate  in  which  the  evolution  of  higher  organic  forms 
would  scarcely  be  rapid  to  an  extremely  diverse  and 
complex  climate  highly  favorable  to  progressive  evolu- 
tion. The  importance  of  these  purely  terrestrial  agencies 
must  not  be  lost  sight  of  when  we  come  to  discuss  other 
agencies  outside  the  earth. 


THE  VARIABILITY  OF  CLIMATE  21 

In  Table  2  the  next  type  of  climatic  sequence  is  geo- 
logic oscillation.  This  means  slow  swings  that  last 
millions  of  years.  At  one  extreme  of  such  an  oscillation 
the  climate  all  over  the  world  is  relatively  monotonous ; 
it  returns,  as  it  were,  toward  the  primeval  conditions  at 
the  beginning  of  the  secular  progression.  At  such  times 
magnolias,  sequoias,  figs,  tree  ferns,  and  many  other 
types  of  subtropical  plants  grew  far  north  in  places  like 
Greenland,  as  is  well  known  from  their  fossil  remains  of 
middle  Cenozoic  time,  for  example.  At  these  same  times, 
and  also  at  many  others  before  such  high  types  of  plants 
had  evolved,  reef-making  corals  throve  in  great  abun- 
dance in  seas  which  covered  what  is  now  Wisconsin, 
Michigan,  Ontario,  and  other  equally  cool  regions.  Today 
these  regions  have  an  average  temperature  of  only  about 
70 °F.  in  the  warmest  month,  and  average  well  below 
freezing  in  winter.  No  reef-making  corals  can  now  live 
where  the  temperature  averages  below  68°F.  The  re- 
semblance of  the  ancient  corals  to  those  of  today  makes 
it  highly  probable  that  they  were  equally  sensitive  to  low 
temperature.  Thus,  in  the  mild  portions  of  a  geologic 
oscillation  the  climate  seems  to  have  been  so  equable  and 
uniform  that  many  plants  and  animals  could  live  1500 
and  at  other  times  even  4000  miles  farther  from  the 
equator  than  now. 

At  such  times  the  lands  in  middle  and  high  latitudes 
were  low  and  small,  and  the  oceans  extended  widely  over 
the  continental  platforms.  Thus  unhampered  ocean  cur- 
rents had  an  opportunity  to  carry  the  heat  of  low  lati- 
tudes far  toward  the  poles.  Under  such  conditions,  es- 
pecially if  the  conception  of  the  great  subequatorial 
continent  of  Gondwana  land  is  correct,  the  trade  winds 
and  the  westerlies  must  have  been  stronger  and  steadier 
than  now.  This  would  not  only  enable  the  westerlies, 


22  CLIMATIC  CHANGES 

which  are  really  south  westerlies,  to  carry  more  heat  than 
now  to  high  latitudes,  but  would  still  further  strengthen 
the  ocean  currents.  At  the  same  time,  the  air  presumably 
contained  an  abundance  of  water  vapor  derived  from 
the  broad  oceans,  and  an  abundance  of  atmospheric 
carbon  dioxide  inherited  from  a  preceding  time  when 
volcanoes  contributed  much  carbon  dioxide  to  the  air. 
These  two  constituents  of  the  atmosphere  may  have 
exercised  a  pronounced  blanketing  effect  whereby  the 
heat  of  the  earth  with  its  long  wave  lengths  was  kept  in, 
although  the  energy  of  the  sun  with  its  shorter  wave 
lengths  was  not  markedly  kept  out.  Thus  everything  may 
have  combined  to  produce  mild  conditions  in  high  lati- 
tudes, and  to  diminish  the  contrast  between  equator  and 
pole,  and  between  summer  and  winter. 

Such  conditions  perhaps  carry  in  themselves  the  seeds 
of  decay.  At  any  rate  while  the  lands  lie  quiet  during  a 
period  of  mild  climate  great  strains  must  accumulate  in 
the  crust  because  of  the  earth's  contraction  and  tidal 
retardation.  At  the  same  time  the  great  abundance  of 
plants  upon  the  lowlying  plains  with  their  mild  climates, 
and  the  marine  creatures  upon  the  broad  continental 
platforms,  deplete  the  atmospheric  carbon  dioxide.  Part 
of  this  is  locked  up  as  coal  and  part  as  limestone  derived 
from  marine  plants  as  well  as  animals.  Then  something 
happens  so  that  the  strains  and  stresses  of  the  crust  are 
released.  The  sea  floors  sink;  the  continents  become 
relatively  high  and  large ;  mountain  ranges  are  formed ; 
and  the  former  plains  and  emergent  portions  of  the 
continental  platforms  are  eroded  into  hills  and  valleys. 
The  large  size  of  the  continents  tends  to  create  deserts 
and  other  types  of  climatic  diversity;  the  presence  of 
mountain  ranges  checks  the  free  flow  of  winds  and  also 
creates  diversity;  the  ocean  currents  are  likewise 


THE  VARIABILITY  OF  CLIMATE  23 

checked,  altered,  and  diverted  so  that  the  flow  of  heat 
from  low  to  high  latitudes  is  diminished.  At  the  same 
time  evaporation  from  the  ocean  diminishes  so  that  a 
decrease  in  water  vapor  combines  with  the  previous  de- 
pletion of  carbon  dioxide  to  reduce  the  blanketing  effect 
of  the  atmosphere.  Thus  upon  periods  of  mild  monotony 
there  supervene  periods  of  complexity,  diversity,  and 
severity.  Turn  to  Table  1  and  see  how  a  glacial  climate 
again  and  again  succeeds  a  time  when  relative  mildness'1, 
prevailed  almost  everywhere.  Or  examine  Fig.  1  and 
notice  how  the  lines  representing  temperatures  go  up  and 
down.  In  the  figure  Schuchert  makes  it  clear  that  when 
the  lands  have  been  large  and  mountain-making  has  been 
important,  as  shown  by  the  high  parts  of  the  lower  shaded 
area,  the  climate  has  been  severe,  as  shown  by  the  descent 
of  the  snow  line,  the  upper  shaded  area.  In  the  diagram 
the  climatic  oscillations  appear  short,  but  this  is  merely 
because  they  have  been  crowded  together,  especially  in 
the  left  hand  or  early  part.  There  an  inch  in  length  may 
represent  a  hundred  million  years.  Even  at  the  right- 
hand  end  an  inch  is  equivalent  to  several  million  years. 

The  severe  part  of  a  climatic  oscillation,  as  well  as  the 
mild  part,  will  be  shown  in  later  chapters  to  bear  in  itself 
certain  probable  seeds  of  decay.  While  the  lands  are 
being  uplifted,  volcanic  activity  is  likely  to  be  vigorous 
and  to  add  carbon  dioxide  to  the  air.  Later,  as  the  moun- 
tains are  worn  down  by  the  many  agencies  of  water, 
wind,  ice,  and  chemical  decay,  although  much  carbon 
dioxide  is  locked  up  by  the  carbonation  of  the  rocks,  the 
carbon  locked  up  in  the  coal  is  set  free  and  increases  the 
carbon  dioxide  of  the  air.  At  the  same  time  the  continents 
settle  slowly  downward,  for  the  earth's  crust  though 
rigid  as  steel  is  nevertheless  slightly  viscous  and  will 
flow  if  subjected  to  sufficiently  great  and  enduring  pres- 


24  CLIMATIC  CHANGES 

sure.  The  area  from  which  evaporation  can  take  place 
is  thereby  increased  because  of  the  spread  of  the  oceans 
over  the  continents,  and  water  vapor  joins  with  the  car- 
bon dioxide  to  blanket  the  earth  and  thus  tends  to  keep  it 
uniformly  warm.  Moreover,  the  diminution  of  the  lands 
frees  the  ocean  currents  from  restraint  and  permits  them 
to  flow  more  freely  from  low  latitudes  to  high.  Thus  in 
the  course  of  millions  of  years  there  is  a  return  toward 
monotony.  Ultimately,  however,  new  stresses  accumulate 
in  the  earth's  crust,  and  the  way  is  prepared  for  another 
great  oscillation.  Perhaps  the  setting  free  of  the  stresses 
takes  place  simply  because  the  strain  at  last  becomes 
irresistible.  It  is  also  possible,  as  we  shall  see,  that  an 
external  agency  sometimes  adds  to  the  strain  and  thereby 
determines  the  time  at  which  a  new  oscillation  shall 
begin. 

In  Table  2  the  types  of  climatic  sequences  which  fol- 
low ''geologic  oscillations"  are  "glacial  fluctuations," 
"orbital  precessions"  and  "historical  pulsations." 
Glacial  fluctuations  and  historical  pulsations  appear  to 
be  of  the  same  type,  except  as  to  severity  and  duration, 
and  hence  may  be  considered  together.  They  will  be 
treated  briefly  here  because  the  theories  as  to  their 
causes  are  outlined  in  the  next  two  chapters.  Oddly 
enough,  although  the  historic  pulsations  lie  much  closer 
to  us  than  do  the  glacial  fluctuations,  they  were  not 
discovered  until  two  or  three  generations  later,  and  are 
still  much  less  known.  The  most  important  feature  of 
both  sequences  is  the  swing  from  a  glacial  to  an  inter- 
glacial  epoch  or  from  the  arsis  or  accentuated  part  of  an 
historical  pulsation  to  the  thesis  or  unaccented  part.  In  a 
glacial  epoch  or  in  the  arsis  of  an  historic  pulsation, 
storms  are  usually  abundant  and  severe,  the  mean  tem- 
perature is  lower  than  usual,  snow  accumulates  in  high 


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26  CLIMATIC  CHANGES 

latitudes  or  upon  lofty  mountains.  For  example,  in  the 
last  such  period  during  the  fourteenth  century,  great 
floods  and  droughts  occurred  alternately  around  the 
North  Sea;  it  was  several  times  possible  to  cross  the 
Baltic  Sea  from  Germany  to  Sweden  on  the  ice,  and  the 
ice  of  Greenland  advanced  so  much  that  shore  ice  caused 
the  Norsemen  to  change  their  sailing  route  between  Ice- 
land and  the  Norse  colonies  in  southern  Greenland.  At 
the  same  time  in  low  latitudes  and  in  parts  of  the  con- 
tinental interior  there  is  a  tendency  toward  diminished 
rainfall  and  even  toward  aridity  and  the  formation  of 
deserts.  In  Yucatan,  for  example,  a  diminution  in  tropi- 
cal rainfall  in  the  fourteenth  century  seems  to  have  given 
the  Mayas  a  last  opportunity  for  a  revival  of  their  decay- 
ing civilization. 

Among  the  climatic  sequences,  glacial  fluctuations  are 
perhaps  of  the  most  vital  import  from  the  standpoint  of 
organic  evolution ;  from  the  standpoint  of  human  history 
the  same  is  true  of  climatic  pulsations.  Glacial  epochs 
have  repeatedly  wiped  out  thousands  upon  thousands  of 
species  and  played  a  part  in  the  origin  of  entirely  new 
types  of  plants  and  animals.  This  is  best  seen  when  the 
life  of  the  Pennsylvanian  is  contrasted  with  that  of  the 
Permian.  An  historic  pulsation  may  wipe  out  an  entire 
civilization  and  permit  a  new  one  to  grow  up  with  a  radi- 
cally different  character.  Hence  it  is  not  strange  that  the 
causes  of  such  climatic  phenomena  have  been  discussed 
with  extraordinary  vigor.  In  few  realms  of  science  has 
there  been  a  more  imposing  or  more  interesting  array  of 
theories.  In  this  book  we  shall  consider  the  more  impor- 
tant of  these  theories.  A  new  solar  or  cyclonic  hypothesis 
and  the  hypothesis  of  changes  in  the  form  and  altitude  of 
the  land  will  receive  the  most  attention,  but  the  other 


THE  VARIABILITY  OF  CLIMATE  27 

chief  hypotheses  are  outlined  in  the  next  chapter,  and  are 
frequently  referred  to  throughout  the  volume. 

Between  glacial  fluctuations  and  historical  pulsations 
in  duration,  but  probably  less  severe  than  either,  come 
orbital  precessions.  These  stand  in  a  group  by  them- 
selves and  are  more  akin  to  seasonal  alternations  than 
to  any  other  type  of  climatic  sequence.  They  must  have 
occurred  with  absolute  regularity  ever  since  the  earth 
began  to  revolve  around  the  sun  in  its  present  elliptical 
orbit.  Since  the  orbit  is  elliptical  and  since  the  sun  is  in 
one  of  the  two  foci  of  the  ellipse,  the  earth's  distance 
from  the  sun  varies.  At  present  the  earth  is  nearest  the 
sun  in  the  northern  winter.  Hence  the  rigor  of  winter  in 
the  northern  hemisphere  is  mitigated,  while  that  of  the 
southern  hemisphere  is  increased.  In  about  ten,  thousand 
years  this  condition  will  be  reversed,  and  in  another  ten,.  Y"\ , 
thousand  the  present  conditions  will  return  once  more. 
Such  climatic  precessions,  as  we  may  here  call  them, 
must  have  occurred  unnumbered  times  in  the  past,  but 
they  do  not  appear  to  have  been  large  enough  to  leave  in 
the  fossils  of  the  rocks  any  traces  that  can  be  distin- 
guished from  those  of  other  climatic  sequences. 

We  come  now  to  Bruckner  periods  and  sunspot  cycles. 
The  Bruckner  periods  have  a  length  of  about  thirty-three 
years.  Their  existence  was  suggested  at  least  as  long  ago 
as  the  days  of  Sir  Francis  Bacon,  whose  statement  about 
them  is  quoted  on  the  flyleaf  of  this  book.  They  have 
since  been  detected  by  a  careful  study  of  the  records  of 
the  time  of  harvest,  vintage,  the  opening  of  rivers  to 
navigation,  and  the  rise  or  fall  of  lakes  like  the  Caspian 
Sea.  In  his  book  on  Klimaschwankungen  seit  1700, 
Bruckner  has  collected  an  uncommonly  interesting  assort- 
ment of  facts  as  to  the  climate  of  Europe  for  more  than 
two  centuries.  More  recently,  by  a  study  of  the  rate  of 


28  CLIMATIC  CHANGES 

growth  of  trees,  Douglass,  in  his  book  on  Climatic  Cycles 
and  Tree  Growth,  has  carried  the  subject  still  further. 
In  general  the  nature  of  the  33-year  periods  seems  to  be 
identical  with  that  of  the  11-  or  12-  year  sunspot  cycle, 
on  the  one  hand,  and  of  historic  pulsations  on  the  other. 
For  a  century  observers  have  noted  that  the  variations 
in  the  weather  which  everyone  notices  from  year  to  year 
seem  to  have  some  relation  to  sunspots.  For  generations, 
however,  the  relationship  was  discussed  without  leading 
to  any  definite  conclusion.  The  trouble  was  that  the  same 
change  was  supposed  to  take  place  in  all  parts  of  the 
world.  Hence,  when  every  sort  of  change  was  found 
somewhere  at  any  given  sunspot  stage,  it  seemed  as 
though  there  could  not  be  a  relationship.  Of  late  years, 
however,  the  matter  has  become  fairly  clear.  The  chief 
conclusions  are,  first,  that  when  sunspots  are  numerous 
the  average  temperature  of  the  earth's  surface  is  lower 
than  normal.  This  does  not  mean  that  all  parts  are  cooler, 
for  while  certain  large  areas  grow  cool,  others  of  less 
extent  become  warm  at  times  of  many  sunspots.  Second, 
at  times  of  many  sunspots  storms  are  more  abundant 
than  usual,  but  are  also  confined  somewhat  closely  to 
certain  limited  tracks  so  that  elsewhere  a  diminution  of 
storminess  may  be  noted.  This  whole  question  is  dis- 
cussed so  fully  in  Earth  and  Sun  that  it  need  not  detain 
us  further  in  this  preliminary  view  of  the  whole  problem 
of  climate.  Suffice  it  to  say  that  a  study  of  the  sunspot 
cycle  leads  to  the  conclusion  that  it  furnishes  a  clue  to 
many  of  the  unsolved  problems  of  the  climate  of  the 
past,  as  well  as  a  key  to  prediction  of  the  future. 

Passing  by  the  seasonal  alternations  which  are  fully 
explained  as  the  result  of  the  revolution  of  the  earth 
around  the  sun,  we  may  merely  point  out  that,  like  the 
daily  vibrations  which  bring  Table  2  to  a  close,  they 


THE  VARIABILITY  OF  CLIMATE  29 

emphasize  the  outstanding  fact  that  the  main  control  of 
terrestrial  climate  is  the  amount  of  energy  received  from 
the  sun.  This  same  principle  is  illustrated  by  pleionian 
migrations.  The  term  "pleion"  comes  from  a  Greek  word 
meaning  "more."  It  was  taken  by  Arctowski  to  desig- 
nate areas  or  periods  where  there  is  an  excess  of  some 
climatic  element,  such  as  atmospheric  pressure,  rainfall, 
or  temperature.  Even  if  the  effect  of  the  seasons  is  elimi- 
nated, it  appears  that  the  course  of  these  various  ele- 
ments does  not  run  smoothly.  As  everyone  knows,  a  period 
like  the  autumn  of  1920  in  the  eastern  United  States  may 
be  unusually  warm,  while  a  succeeding  period  may  be 
unseasonably  cool.  These  departures  from  the  normal 
show  a  certain  rough  periodicity.  For  example,  there  is 
evidence  of  a  period  of  about  twenty-seven  days,  corre- 
sponding to  the  sun 's  rotation  and  formerly  supposed  to 
be  due  to  the  moon's  revolution  which  occupies  almost 
the  same  length  of  time.  Still  other  periods  appear  to 
have  an  average  duration  of  about  three  months  and  of 
between  two  and  three  years.  Two  remarkable  discoveries 
have  recently  been  made  in  respect  to  such  pleions.  One 
is  that  a  given  type  of  change  usually  occurs  simulta- 
neously in  a  number  of  well-defined  but  widely  separated 
centers,  while  a  change  of  an  opposite  character  arises 
in  another  equally  well-defined,  but  quite  different,  set 
of  centers.  In  general,  areas  of  high  pressure  have  one 
type  of  change  and  areas  of  low  pressure  the  other  type. 
So  systematic  are  these  relationships  and  so  completely 
do  they  harmonize  in  widely  separated  parts  of  the  earth, 
that  it  seems  certain  that  they  must  be  due  to  some  out- 
side cause,  which  in  all  probability  can  be  only  the  sun. 
The  second  discovery  is  that  pleions,  when  once  formed, 
travel  irregularly  along  the  earth's  surface.  Their  paths 
have  not  yet  been  worked  out  in  detail,  but  a  general 


30  CLIMATIC  CHANGES 

migration  seems  well  established.  Because  of  this,  it  is 
probable  that  if  unusually  warm  weather  prevails  in  one 
part  of  a  continent  at  a  given  time,  the  * '  thermo-pleion, " 
or  excess  of  heat,  will  not  vanish  but  will  gradually  move 
away  in  some  particular  direction.  If  we  knew  the  path 
that  it  would  follow  we  might  predict  the  general  tem- 
perature along  its  course  for  some  months  in  advance. 
The  paths  are  often  irregular,  and  the  pleions  frequently 
show  a  tendency  to  break  up  or  suddenly  revive.  Prob- 
ably this  tendency  is  due  to  variations  in  the  sun.  When 
v  the  sun  is  highly  variable,  the  pleions  are  numerous  and 
strong,  and  extremes  of  weather  are  frequent.  Taken  as 
a  whole  the  pleions  offer  one  of  the  most  interesting  and 
hopeful  fields  not  only  for  the  student  of  the  causes  of 
climatic  variations,  but  for  the  man  who  is  interested  in 
the  practical  question  of  long-range  weather  forecasts. 
Like  many  other  climatic  phenomena  they  seem  to  repre- 
sent the  combined  effect  of  conditions  in  the  sun  and 
upon  the  earth  itself. 

The  last  of  the  climatic  sequences  which  require  ex- 
planation is  the  cyclonic  vacillations.  These  are  familiar 
to  everyone,  for  they  are  the  changes  of  weather  which 
occur  at  intervals  of  a  few  days,  or  a  week  or  two,  at  all 
seasons,  in  large  parts  of  the  United  States,  Europe, 
Japan,  and  some  of  the  other  progressive  parts  of  the 
earth.  They  do  not,  however,  occur  with  great  frequency 
in  equatorial  regions,  deserts,  and  many  other  regions. 
Up  to  the  end  of  the  last  century,  it  was  generally  sup- 
posed that  cyclonic  storms  were  purely  terrestrial  in 
origin.  Without  any  adequate  investigation  it  was  as- 
sumed that  all  irregularities  in  the  planetary  circulation 
of  the  winds  arise  from  an  irregular  distribution  of  heat 
due  to  conditions  within  or  upon  the  earth  itself.  These 
irregularities  were  supposed  to  produce  cyclonic  storms 


THE  VARIABILITY  OF  CLIMATE  31 

in  certain  limited  belts,  but  not  in  most  parts  of  the 
world.  Today  this  view  is  being  rapidly  modified.  Un- 
doubtedly, the  irregularities  due  to  purely  terrestrial 
conditions  are  one  of  the  chief  contributory  causes  of 
storms,  but  it  begins  to  appear  that  solar  variations  also 
play  a  part.  It  has  been  found,  for  example,  that  not 
only  the  mean  temperature  of  the  earth's  surface  varies 
in  harmony  with  the  sunspot  cycle,  but  that  the  frequency 
and  severity  of  storms  vary  in  the  same  way.  Moreover, 
it  has  been  demonstrated  that  the  sun's  radiation  is  not 
constant,  but  is  subject  to  innumerable  variations.  This 
does  not  mean  that  the  sun's  general  temperature  varies, 
but  merely  that  at  some  times  heated  gases  are  ejected; 
rapidly  to  high  levels  so  that  a  sudden  wave  of  energy\jJvx 
strikes  the  earth.  Thus,  the  present  tendency  is  to  believe  ^> 
that  the  cyclonic  variations,  the  changes  of  weather 
which  come  and  go  in  such  a  haphazard,  irresponsible 
way,  are/partly  due  to  causes  pertaining  to  the  earth 
itself  and  partly  to  the  sun. 

From  this  rapid  survey  of  the  types  of  climatic  se- 
quences, it  is  evident  that  they  may  be  divided  into  four      / 
great  groups.  First  comes  cosmic  uniformity,  one  of  the    / 
most  marvelous  and  incomprehensible  of  all  known  facts.  / 
We  simply  have  no  explanation  which  is  in  any  respect 
adequate.  Next  come  secular  progression  and  geologic 
oscillations,  two  types  of  change  which  seem  to  be  due 
mainly  to  purely  terrestrial  causes,  that  is,  to  changes  in 
the  lands,  the  oceans,  and  the  air.  The  general  tendency 
of  these  changes  is  toward  complexity  and  diversity,  thus 
producing  progression,  but  they  are  subject  to  frequent 
reversals  which  give  rise  to  oscillations  lasting  millions 
of  years.  The  processes  by  which  the  oscillations  take 
place  are  fully  discussed  in  this  book.  Nevertheless,  be- 
cause they  are  fairly  well  understood,  they  are  deferred 


32  CLIMATIC  CHANGES 

until  after  the  third  group  of  sequences  has  been  dis- 
cussed. This  group  includes  glacial  fluctuations,  historic 
pulsations,  Bruckner  periods,  sunspot  cycles,  pleionian 
migrations,  and  cyclonic  vacillations.  The  outstanding 
fact  in  regard  to  all  of  these  is  that  while  they  are  greatly 
modified  by  purely  terrestrial  conditions,  they  seem  to 
owe  their  origin  to  variations  in  the  sun.  They  form  the 
chief  subject  of  Earth  and  Sun  and  in  their  larger  phases 
are  the  most  important  topic  of  this  book  also.  The  last 
group  of  sequences  includes  orbital  precessions,  seasonal 
alternations,  and  daily  variations.  These  may  be  re- 
garded as  purely  solar  in  origin.  Yet  their  influence,  like 
that  of  each  of  the  other  groups,  is  much  modified  by  the 
earth's  own  conditions.  Our  main  problem  is  to  separate 
and  explain  the  two  great  elements  in  climatic  changes, 
— the  effects  of  the  sun,  on  the  one  hand,  and  of  the  earth 
on  the  other. 


CHAPTER  III 
HYPOTHESES  OF  CLIMATIC  CHANGE 

THE  next  step  in  our  study  of  climate  is  to  review 
the  main  hypotheses  as  to  the  causes  of  glacia- 
tion.  These  hypotheses  apply  also  to  other  types 
of  climatic  changes.  We  shall  concentrate  on  glacial 
periods,  however,  not  only  because  they  are  the  most 
dramatic  and  well-known  types  of  change,  but  because 
they  have  been  more  discussed  than  any  other  and  have 
also  had  great  influence  on  evolution.  Moreover,  they 
stand  near  the  middle  of  the  types  of  climatic  sequences, 
and  an  understanding  of  them  does  much  to  explain  the 
others.  In  reviewing  the  various  theories  we  shall  not 
attempt  to  cover  all  the  ground,  but. shall  merely  state 
the  main  ideas  of  the  few  theories  which  have  had  an 
important  influence  upon  scientific  thought. 

The  conditions  which  any  satisfactory  climatic  hy- 
pothesis must  satisfy  are  briefly  as  follows : 

(1)  Due  weight  must  be  given  to  the  fact  that  changes 
of  climate  are  almost  certainly  due  to  the  combined  effect 
of  a  variety  of  causes,  both  terrestrial  and  solar  or 
cosmic. 

(2)  Attention  must  also  be  paid  to  both  sides  in  the 
long  controversy  as  to  whether  glaciation  is  due  pri- 
marily to  a  diminution  in  the  earth 's  supply  of  heat  or  to 
a  redistribution  of  the  heat  through  changes  in  atmos- 
pheric and   oceanic  circulation.  At  present  the   great 


34  CLIMATIC  CHANGES 

majority  of  authorities  are  on  the  side  of  a  diminution  of 
heat,  but  the  other  view  also  deserves  study. 

(3)  A  satisfactory  hypothesis  must  explain  the  fre- 
quent  synchronism   between   two   great   types   of  phe- 
nomena; first,  movements  of  the  earth's  crust  whereby 
continents  are  uplifted  and  mountains  upheaved;  and, 
second,    great   changes    of   climate   which   are    usually 
marked  by  relatively  rapid  oscillations  from  one  extreme 
to  another. 

(4)  No  hypothesis  can  find  acceptance  unless  it  satis- 
fies the  somewhat  exacting  requirements  of  the  geological 
record,  with  its  frequent  but  irregular  repetition  of  long, 
mild  periods,  relatively  cool  or  intermediate  periods  like 
the  present,  and  glacial  periods  of  more  or  less  severity 
and  perhaps  accompanying  the  more  or  less  widespread 
uplifting  of  continents.  At  least  during  the  later  glacial 
periods  the  hypothesis  must  explain  numerous  climatic 
epochs  and  stages   superposed  upon   a   single  general 
period  of  continental  upheaval.  Moreover,  although  his- 
torical geology  demands  cycles  of  varied  duration  and 
magnitude,  it  does  not  furnish  evidence  of  any  rigid 
periodicity  causing  the  cycles  to  be  uniform  in  length  or 
intensity. 

(5)  Most  important  of  all,  a  satisfactory  explanation 
of  climatic  changes  and  crustal  deformation  must  take 
account  of  all  the  agencies  which  are  now  causing  similar 
phenomena.  Whether  any  other  agencies  should  be  con- 
sidered is  open  to  question,  although  the  relative  im- 
portance of  existing  agencies  may  have  varied. 

I.  C roll's  Eccentricity  Theory.  One  of  the  most  in- 
genious and  most  carefully  elaborated  scientific  hy- 
potheses is  CrolPs1  precessional  hypothesis  as  to  the 
effect  of  the  earth's  own  motions.  So  well  was  this  worked 

i  James  Croll :  Climate  and  Time,  1876. 


HYPOTHESES  OF  CLIMATIC  CHANGES          35 

out  that  it  was  widely  accepted  for  a  time  and  still  finds  a 
place  in  popular  but  unscientific  books,  such  as  Wells' 
Outline  of  History,  and  even  in  scientific  works  like 
Wright's  Quaternary  Ice  Age.  The  gist  of  the  hypothe- 
sis has  already  been  given  in  connection  with  the  type  of 
climatic  sequence  known  as  orbital  precessions.  The  earth 
is  93  million  miles  away  from  the  sun  in  January  and  97 
million  in  July.  The  earth's  axis  "processes,"  however, 
just  as  does  that  of  a  spinning  top.  Hence  arises  what  is 
known  as  the  precession  of  the  equinoxes,  that  is,  a        O\o 
steady  change  in  the  season  at  which  the  earth  is  in  peji--^ 
helion,  or  nearest  to  the  sun.  In  the  course  of  21,000  years 
the  time  of  perihelion  varies  from  early  in  January 
through  the  entire  twelve  months  and  back  to  January. 
Moreover,  the  earth's  orbit  is  slightly  more  elliptical  at   \    >~ 
certain  periods  than  at  others,  for  the  planets  sometimes    X. 
become  bunched  so  that  they  all  pull  the  earth  in  one/ 
direction.  Hence,  once  in  about  one  hundred  thousand 
years  the  effect  of  the  elliptical  shape  of  the  earth's  orbit 
is  at  a  maximum. 

Croll  argued  that  these  astronomical  changes  must 
alter  the  earth's  climate,  especially  by  their  effect  on 
winds  and  ocean  currents.  His  elaborate  argument  con- 
tains a  vast  amount  of  valuable  material.  Later  investi- 
gation, however,  seems  to  have  proven  the  inadequacy  of 
his  hypothesis.  In  the  first  place,  the  supposed  cause  does 
not  seem  nearly  sufficient  to  produce  the  observed  results. 
Second,  CrolPs  hypothesis  demands  that  glaciation  in  the 
northern  and  southern  hemisphere  take  place  alternately. 
A  constantly  growing  collection  of  facts,  however,  indi- 
cates that  glaciation  does  not  occur  in  the  two  hemi- 
spheres alternately,  but  at  the  same  time.  Third,  the 
hypothesis  calls  for  the  constant  and  frequent  repetition 
of  glaciation  at  absolutely  regular  intervals.  The  geo- 


36  CLIMATIC  CHANGES 

logical  record  shows  no  such  regularity,  for  sometimes 
several  glacial  epochs  follow  in  relatively  close  succes- 
sion at  irregular  intervals  of  perhaps  fifty  to  two  hun- 
dred thousand  years,  and  thus  form  a  glacial  period ;  and 
then  for  millions  of  years  there  are  none.  Fourth,  the 
eccentricity  hypothesis  provides  no  adequate  explanation 
for  the  glacial  stages  or  subepochs,  the  historic  pulsa- 
tions, and  the  other  smaller  climatic  variations  which  are 
superposed  upon  glacial  epochs  and  upon  one  another  in 
bewildering  confusion.  In  spite  of  these  objections,  there 
can  be  little  question  that  the  eccentricity  of  the  earth's 
orbit  and  the  precession  of  the  equinoxes  with  the  result- 
ing change  in  the  season  of  perihelion  must  have^scime 
climatic  effect.  Hence  Croll's  theory  deserves  a  perma- 
nent though  minor  place  in  any  full  discussion  of  the 
causes  of  climatic  changes. 

II.  The  Carbon  Dioxide  Theory.  At  about  the  time 
that  the  eccentricity  theory  was  being  relegated  to  a 
minor  niche,  a  new  theory  was  being  developed  which 
soon  exerted  a  profound  influence  upon  geological 
thought.  Chamberlin,2  adopting  an  idea  suggested  by 

2  T.  C.  Chamberlin :  An  attempt  to  frame  a  working  hypothesis  of  the 
cause  of  glacial  periods  on  an  atmospheric  basis;  Jour.  Geol.,  Vol.  VII, 
1899,  pp.  545-584,  667-685,  757-787. 

T.  C.  Chamberlin  and  E.  D.  Salisbury:  Geology,  Vol.  II,  1906,  pp.  93- 
106,  655-677,  and  Vol.  Ill,  pp.  432-446. 

S.  Arrhenius  (Kosmische  Physik,  Vol.  II,  1903,  p.  503)  carried  out  some 
investigations  on  carbon  dioxide  which  have  had  a  pronounced  effect  on 
later  conclusions. 

F.  Freeh  adopted  Arrhenius'  idea  and  developed  it  in  a  paper  entitled 
Ueber  die  Klima-Aenderungen  der  Geologischen  Vergangenheit.  Compte 
Eendu,  Tenth  (Mexico)  Congr.  Geol.  Intern.,  1907  (=1908),  pp.  299-325. 

The  exact  origin  of  the  carbon  dioxide  theory  has  been  stated  so  variously 
that  it  seems  worth  while  to  give  the  exact  facts.  Prompted  by  the  sug- 
gestion of  Tyndall  that  glaciation  might  be  due  to  depletion  of  atmospheric 
carbon  dioxide,  Chamberlin  worked  up  the  essentials  of  his  early  views 
before  he  saw  any  publication  from  Arrhenius,  to  whom  the  idea  has  often 
been  attributed.  In  1895  or  earlier  Chamberlin  began  to  give  the  carbon 
dioxide  hypothesis  to  his  students  and  to  discuss  it  before  local  scientific 


HYPOTHESES  OF  CLIMATIC  CHANGES         37 

Tyndall,  fired  the  imagination  of  geologists  by  his  skill- 
ful exposition  of  the  part  played  by  carbon  dioxide  in 
causing  climatic  changes.  Today  this  theory  is  probably 
more  widely  accepted  than  any  other.  We  have  already 
seen  that  the  amount  of  carbon  dioxide  gas  in  the  at- 
mosphere has  a  decided  climatic  importance.  Moreover, 
there  can  be  little  doubt  that  the  amount  of  that  gas  in 
the  atmosphere  varies  from  age  to  age  in  response  to  the 
extent  to  which  it  is  set  free  by  volcanoes,  consumed  by 
plants,  combined  with  rocks  in  the  process  of  weathering, 
dissolved  in  the  ocean  or  locked  up  in  the  form  of  coal 
and  limestone.  The  main  question  is  whether  such  varia- 
tions can  produce  changes  so  rapid  as  glacial  epochs  and 
historical  pulsations. 

Abundant  evidence  seems  to  show  that  the  degree  to 
which  the  air  can  be  warmed  by  carbon  dioxide  is  sharply 
limited.  Humphreys,  in  his  excellent  book  on  the  Physics 
of  the  Air,  calculates  that  a  layer  of  carbon  dioxide  forty 
centimeters  thick  has  practically  as  much  blanketing 
effect  as  a  layer  indefinitely  thicker.  In  other  words,  forty 
centimeters  of  carbon  dioxide,  while  having  no  appreci- 

bodies.  In  1897  he  prepared  a  paper  on  "A  Group  of  Hypotheses  Bearing 
on  Climatic  Changes,"  Jour.  Geol.,  Vol.  V  (1897),  to  be  read  at  the  meeting 
of  the  British  Association  at  Toronto,  basing  his  conclusions  on  Tyndall 's 
determination  of  the  competency  of  carbon  dioxide  as  an  absorber  of  heat 
radiated  from  the  earth.  He  had  essentially  completed  this  when  a  paper  by 
Arrhenius  "On  the  influence  of  carbonic  acid  in  the  air  upon  the  tem- 
perature of  the  ground,"  Phil.  Mag.,  1896,  pp.  237-276,  first  came  to  his 
attention.  Chamberlin  then  changed  his  conservative,  tentative  statement  of 
the  functions  of  carbon  dioxide  to  a  more  sweeping  one  based  on  Arrhenius' 
very  definite  quantitative  deductions  from  Langley's  experiments.  Both 
Langley  and  Arrhenius  were  then  in  the  ascendancy  of  their  reputations 
and  seemingly  higher  authorities  could  scarcely  have  been  chosen,  nor  a 
finer  combination  than  experiment  and  physico-mathematical  development. 
Arrhenius'  deductions  were  later  proved  to  have  been  overstrained,  while 
Langley's  interpretation  and  even  his  observations  were  challenged.  Cham- 
berlin's  latest  views  are  more  like  his  earlier  and  more  conservative  state- 
ment. 


38  CLIMATIC  CHANGES 

able  effect  on  sunlight  coming  toward  the  earth,  would 
filter  out  and  thus  retain  in  the  atmosphere  all  the  out- 
going terrestrial  heat  that  carbon  dioxide  is  capable  of 
absorbing.  Adding  more  would  be  like  adding  another 
filter  when  the  one  in  operation  has  already  done  all  that 
that  particular  kind  of  filter  is  capable  of  doing.  Accord- 
ing to  Humphreys '  calculations,  a  doubling  of  the  carbon 
dioxide  in  the  air  would  in  itself  raise  the  average  tem- 
perature about  1.3° C.  and  further  carbon  dioxide  would 
have  practically  no  effect.  Reducing  the  present  supply 
by  half  would  reduce  the  temperature  by  essentially  the 
same  amount. 

The  effect  must  be  greater,  however,  than  would  ap- 
pear from  the  figures  given  above,  for  any  change  in 
temperature  has  an  effect  on  the  amount  of  water  vapor, 
which  in  turn  causes  further  changes  of  temperature. 
Moreover,  as  Chamberlin  points  out,  it  is  not  clear 
whether  Humphreys  allows  for  the  fact  that  when  the 
40  centimeters  of  C02  nearest  the  earth  has  been  heated 
by  terrestrial  radiation,  it  in  turn  radiates  half  its  heat 
outward  and  half  inward.  The  outward  half  is  all  ab- 
sorbed in  the  next  layer  of  carbon  dioxide,  and  so  on. 
The  process  is  much  more  complex  than  this,  but  the  end 
result  is  that  even  the  last  increment  of  C02,  that  is,  the 
outermost  portions  in  the  upper  atmosphere,  must  ap- 
parently absorb  an  infinitesimally  small  amount  of  heat. 
This  fact,  plus  the  effect  of  water  vapor,  would  seem  to 
indicate  that  a  doubling  or  halving  of  the  amount  of  C02 
would  have  an  effect  of  more  than  1.3°C.  A  change  of 
even  2°C.  above  or  below  the  present  level  of  the  earth's 
mean  temperature  would  be  of  very  appreciable  climatic 
significance,  for  it  is  commonly  believed  that  during  the 
height  of  the  glacial  period  the  mean  temperature  was 
only  5°  to  8°C.  lower  than  now. 


HYPOTHESES  OF  CLIMATIC  CHANGES         39 

Nevertheless,  variations  in  atmospheric  carbon  dioxide 
do  not  necessarily  seem  competent  to  produce  the  rela- 
tively rapid  climatic  fluctuations  of  glacial  epochs  and 
historic  pulsations  as  distinguished  from  the  longer 
swings  of  glacial  periods  and  geological  eras.  In  Cham- 
berlin's  view,  as  in  ours,  the  elevation  of  the  land,  the 
modification  of  the  currents  of  the  air  and  of  the  ocean, 
and  all  that  goes  with  elevation  as  a  topographic  agency 
constitute  a  primary  cause  of  climatic  changes.  A  special 
effect  of  this  is  the  removal  of  carbon  dioxide  from  the 
air  by  the  enhanced  processes  of  weathering.  This,  as  he 
carefully  states,  is  a  very  slow  process,  and  cannot  of 
itself  lead  to  anything  so  sudden  as  the  oncoming  of 
glaciation.  But  here  comes  Chamberlin  's  most  distinctive 
contribution  to  the  subject,  namely,  the  hypothesis  that 
changes  in  atmospheric  temperature  arising  from  varia- 
tions in  atmospheric  carbon  dioxide  are  able  to  cause  a 
reversal  of  the  deep-sea  oceanic  circulation. 

According  to  Chamberlin 's  view,  the  ordinary  oceanic 
circulation  of  the  greater  part  of  geological  time  was 
the  reverse  of  the  present  circulation.  Warm  water  de- 
scended to  the  ocean  depths  in  low  latitudes,  kept  its  heat 
while  creeping  slowly  poleward,  and  rose  in  high  lati- 
tudes producing  the  warm  climate  which  enabled  corals, 
for  example,  to  grow  in  high  latitudes.  Chamberlin  holds  */t*^ 
this  opinion  largely  because  there  seems  to  him  to  be  noC'  £\ 
other  reasonable  way  to  account  for  the  enormously  long^  (*- 
warm  periods  when  heat-loving  forms  of  life  lived  in 
what  are  now  polar  regions  of  ice  and  snow.  He  explains 
this  reversed  circulation  by  supposing  that  an  abundance 
of  atmospheric  carbon  dioxide,  together  with  a  broad 
distribution  of  the  oceans,  made  the  atmosphere  so  warm 
that  the  evaporation  in  low  latitudes  was  far  more  rapid 
than  now.  Hence  the  surface  water  of  the  ocean  became 


40  CLIMATIC  CHANGES 

a  relatively  concentrated  brine.  Such  a  brine  is  heavy 
and  tends  to  sink,  thereby  setting  up  an  oceanic  circula- 
tion the  reverse  of  that  which  now  prevails.  At  present 
the  polar  waters  sink  because  they  are  cold  and  hence 
contract.  Moreover,  when  they  freeze  a  certain  amount 
of  salt  leaves  the  ice  and  thereby  increases  the  salinity 
of  the  surrounding  water.  Thus  the  polar  water  sinks 
to  the  depths  of  the  ocean,  its  place  is  taken  by  warmer 
and  lighter  water  from  low  latitudes  which  moves  pole- 
ward along  the  surface,  and  at  the  same  time  the  cold 
water  of  the  ocean  depths  is  forced  equatorward  below 
the  surface.  But  if  the  equatorial  waters  were  so  concen- 
trated that  a  steady  supply  of  highly  saline  water  kept 
descending  to  low  levels,  the  direction  of  the  circulation 
would  have  to  be  reversed.  The  time  when  this  would 
occur  would  depend  upon  the  delicate  balance  between 
the  downward  tendencies  of  the  cold  polar  water  and  of 
the  warm  saline  equatorial  water. 

Suppose  that  while  such  a  reversed  circulation  pre- 
vailed, the  atmospheric  C02  should  be  depleted,  and  the 
air  cooled  so  much  that  the  concentration  of  the  equa- 
torial waters  by  evaporation  was  no  longer  sufficient  to 
cause  them  to  sink.  A  reversal  would  take  place,  the 
present  type  of  circulation  would  be  inaugurated,  and 
the  whole  earth  would  suffer  a  chill  because  the  sur- 
face of  the  ocean  would  become  cool.  The  cool  surface- 
water  would  absorb  carbon  dioxide  faster  than  the  pre- 
vious warm  water  had  done,  for  heat  drives  off  gases 
from  water.  This  would  hasten  the  cooling  of  the  at- 
mosphere still  more,  not  only  directly  but  by  diminishing 
the  supply  of  atmospheric  moisture.  The  result  would  be 
glaciation.  But  ultimately  the  cold  waters  of  the  higher 
latitudes  would  absorb  all  the  carbon  dioxide  they  could 
hold,  the  slow  equatorward  creep  would  at  length  permit 


HYPOTHESES  OF  CLIMATIC  CHANGES          41 

the  cold  water  to  rise  to  the  surface  in  low  latitudes. 
There  the  warmth  of  the  equatorial  sun  and  the  depleted 
supply  of  carbon  dioxide  in  the  air  would  combine  to 
cause  the  water  to  give  up  its  carbon  dioxide  once  more. 
If  the  atmosphere  had  been  sufficiently  depleted  by  that 
time,  the  rising  waters  in  low  latitudes  might  give  up 
more  carbon  dioxide  than  the  cold  polar  waters  absorbed. 
Thus  the  atmospheric  supply  would  increase,  the  air 
would  again  grow  warm,  and  a  tendency  toward  de- 
glaciation,  or  toward  an  inter-glacial  condition  would 
arise.  At  such  times  the  oceanic  circulation  is  not  sup- 
posed to  have  been  reversed,  but  merely  to  have  been 
checked  and  made  slower  by  the  increasing  warmth. 
Thus  inter-glacial  conditions  like  those  of  today,  or  even 
considerably  warmer,  are  supposed  to  have  been  pro- 
duced with  the  present  type  of  circulation. 

The  emission  of  carbon  dioxide  in  low  latitudes  could 
not  permanently  exceed  the  absorption  in  high  latitudes. 
After  the  present  type  of  circulation  was  finally  estab- 
lished, which  might  take  tens  of  thousands  of  years,  the 
two  would  gradually  become  equal.  Then  the  conditions 
which  originally  caused  the  oceanic  circulation  to  be 
reversed  would  again  destroy  the  balance;  the  atmos- 
pheric carbon  dioxide  would  be  depleted;  the  air  would 
grow  cooler;  and  the  cycle  of  glaciation  would  be  re- 
peated. Each  cycle  would  be  shorter  than  the  last,  for  not 
only  would  the  swings  diminish  like  those  of  a  pendulum, 
but  the  agencies  that  were  causing  the  main  depletion  of 
the  atmospheric  carbon  dioxide  would  diminish  in  inten- 
sity. Finally  as  the  lands  became  lower  through  erosion 
and  submergence,  and  as  the  processes  of  weathering 
became  correspondingly  slow,  the  air  would  gradually  be 
able  to  accumulate  carbon  dioxide ;  the  temperature  would 
increase;  and  at  length  the  oceanic  circulation  would  be 


42  CLIMATIC  CHANGES 

reversed  again.  When  the  warm  saline  waters  of  low  lati- 
tudes finally  began  to  sink  and  to  set  up  a  flow  of  warm 
water  poleward  in  the  depths  of  the  ocean,  a  glacial 
period  would  definitely  come  to  an  end. 

This  hypothesis  has  been  so  skillfully  elaborated,  and 
contains  so  many  important  elements  that  one  can 
scarcely  study  it  without  profound  admiration.  We  be- 
lieve that  it  is  of  the  utmost  value  as  a  step  toward  the 
truth,  and  especially  because  it  emphasizes  the  great 
function  of  oceanic  circulation.  Nevertheless,  we  are 
unable  to  accept  it  in  full  for  several  reasons,  which 
may  here  be  stated  very  briefly.  Most  of  them  will  be  dis- 
cussed fully  in  later  pages. 

(1)  While  a  reversal  of  the  deep-sea  circulation  would 
undoubtedly  be  of  great  climatic  importance  and  would 
produce  a  warm  climate  in  high  latitudes,  we  see  no 
direct  evidence  of  such  a  reversal.  It  is  equally  true  that 
there  is  no  conclusive  evidence  against  it,  and  the  possi- 
bility of  a  reversal  must  not  be  overlooked.  There  seem, 
however,  to  be  other  modifications  of  atmospheric  and 
oceanic  circulation  which  are  able  to  produce  the  ob- 
served results. 

(2)  There  is  much,  and  we  believe  conclusive,  evidence 
that  a  mere  lowering  of  temperature  would  not  produce 
glaciation.  What  seems  to  be  needed  is  changes  in  atmos- 
pheric   circulation    and    in    precipitation.    The    carbon 
dioxide  hypothesis  has  not  been  nearly  so  fully  developed 
on  the  meteorological  side  as  in  other  respects. 

(3)  The  carbon  dioxide  hypothesis  seems  to  demand 
that  the  oceans  should  have  been  almost  as  saline  as  now 
in  the  Proterozoic  era  at  the  time  of  the  first  known 
glaciation.  Chamberlin  holds  that  such  was  the  case,  but 
the  constant  supply  of  saline  material  brought  to  the 
ocean  by  rivers  and  the  relatively  small  deposition  of 


HYPOTHESES  OF  CLIMATIC  CHANGES         43 

such  material  on  the  sea  floor  seem  to  indicate  that  the 
early  oceans  must  have  been  much  fresher  than  those  of 
today. 

(4)  The  carbon  dioxide  hypothesis  does  not  attempt 
to  explain  minor  climatic  fluctuations  such  as  post-glacial 
stages  and  historic  pulsations,  but  these  appear  to  be  of 
the   same   nature   as   glacial   epochs,   differing  only  in 
degree. 

(5)  Another  reason  for  hesitation  in   accepting  the 
carbon  dioxide  hypothesis  as  a  full  explanation  of  glacial 
fluctuations  is  the  highly  complex  and  non-observational 
character  of  the  explanation  of  the  alternation  of  glacial 
and  inter-glacial  epochs  and  of  their  constantly  decreas- 
ing length. 

(6)  Most  important  of  all,  a  study  of  the  variations  of 
weather  and  of  climate  as  they  are  disclosed  by  present 
records  and  by  the  historic  past  suggests  that  there  are 
now  in  action  certain  other  causes  which  are  competent 
to  explain  glaciation  without  recourse  to  a  process  whose 
action  is  beyond  the  realm  of  observation. 

These  considerations  lead  to  the  conclusion  that  the 
carbon  dioxide  hypothesis  and  the  reversal  of  the  oceanic 
circulation  should  be  regarded  as  a  tentative  rather  than 
a  final  explanation  of  glaciation.  Nevertheless,  the  action 
of  carbon  dioxide  seems  to  be  an  important  factor  in  pro- 
ducing the  longer  oscillations  of  climate  from  one  geo- 
logical era  to  another.  It  probably  plays  a  considerable 
part  in  preparing  the  way  for  glacial  periods  and  in 
making  it  possible  for  other  factors  to  produce  the  more 
rapid  changes  which  have  so  deeply  influenced  organic 
evolution. 

III.  The  Form  of  the  Land.  Another  great  cause  of 
climatic  change  consists  of  a  group  of  connected  phe- 
nomena dependent  upon  movements  of  the  earth's  crust. 


44  CLIMATIC  CHANGES 

As  to  the  climatic  potency  of  changes  in  the  lands  there 
is  practical  agreement  among  students  of  climatology 
and  glaciation.  That  the  height  and  extent  of  the  conti- 
nents, the  location,  size,  and  orientation  of  mountain 
ranges,  and  the  opening  and  closing  of  oceanic  gateways 
at  places  like  Panama,  and  the  consequent  diversion  of 
V^~trceanic  currents,  exert  a  profound  effect  upon  climate 
can  scarcely  be  questioned.  Such  changes  may  be  intro- 
duced rapidly,  but  their  disappearance  is  usually  slow 
compared  with  the  rapid  pulsations  to  which  climate  has 
been  subject  during  historic  times  and  during  stages  of 
glacial  retreat  and  advance,  or  even  in  comparison  with 
the  epochs  into  which  the  Pleistocene,  Permian,  and 
perhaps  earlier  glacial  periods  have  been  divided.  Hence, 
while  crustal  movements  appear  to  be  more  important 
than  the  eccentricity  of  the  earth's  orbit  or  the  amount  of 
carbon  dioxide  in  the  air,  they  do  not  satisfactorily  ex- 
plain glacial  fluctuations,  historic  pulsations,  and  espe- 
cially the  present  little  cycles  of  climatic  change.  All 
these  changes  involve  a  relatively  rapid  swing  from  one 
extreme  to  another,  while  an  upheaval  of  a  continent, 
which  is  at  best  a  slow  geologic  process,  apparently 
cannot  be  undone  for  a  long,  long  time.  Hence  such  an 
upheaval,  if  acting  alone,  would  lead  to  a  relatively  long- 
lived  climate  of  a  somewhat  extreme  type.  It  would  help 
to  explain  the  long  swings,  or  geologic  oscillations  be- 
tween a  mild  and  uniform  climate  at  one  extreme,  and  a 
complex  and  varied  climate  at  the  other,  but  it  would  not 
explain  the  rapid  climatic  pulsations  which  are  closely 
associated  with  great  movements  of  the  earth's  crust.  It 
might  prepare  the  way  for  them,  but  could  not  cause 
them.  That  this  conclusion  is  true  is  borne  out  by  the  fact 
that  vast  mountain  ranges,  like  those  at  the  close  of  the 
Jurassic  and  Cretaceous,  are  upheaved  without  bringing 


HYPOTHESES  OF  CLIMATIC  CHANGES         45 

on  glacial  climates.  Moreover,  the  marked  Permian  ice^ 
age  follows  long  after  the  birth  of  the  Hercynian  Moun- 
tians  and  before  the  rise  of  others  of  later  Permian 
origin. 

IV.  The  Volcanic  Hypothesis.  In  the  search  for  some 
cause  of  climatic  change  which  is  highly  efficient  and  yet 
able  to  vary  rapidly  and  independently,  Abbot,  Fowle, 
Humphreys,  and  others,3  have  concluded  that  volcanic 
eruptions  are  the  missing  agency.  In  Physics  of  the  Air, 
Humphreys  gives  a  careful  study  of  the  effect  of  vol- 
canic dust  upon  terrestrial  temperature.  He  begins  with 
a  mathematical  investigation  of  the  size  of  dust  particles, 
and  their  quantity  after  certain  eruptions.  He  demon- 
strates tkat  the  power  of  such  particles  to  deflect  light  of 
short  wave-lengths  coming  from  the  sun  is  perhaps  thirty 
times  more  than  their  power  to  retain  the  heat  radiated 
in  long  waves  from  the  earth.  Hence  it  is  estimated  that 
if  a  Krakatoa  were  to  belch  forth  dust  every  year  or 
two,  the  dust  veil  might  cause  a  reduction  of  about  6°C. 
in  the  earth's  surface  temperature.  As  in  every  such  com- 
plicated problem,  some  of  the  author's  assumptions  are 
open  to  question,  but  this  touches  their  quantitative  and 
not  their  qualitative  value.  It  seems  certain  that  if  vol- 
canic explosions  were  frequent  enough  and  violent 
enough,  the  temperature  of  the  earth's  surface  would  be 
considerably  lowered. 

Actual  observation  supports  this  theoretical  conclu- 
sion. Humphreys  gathers  together  and  amplifies  all  that 
he  and  Abbot  and  Fowle  have  previously  said  as  to  obser- 
vations of  the  sun's  thermal  radiation  by  means  of  the 

3C.  G.  Abbot  and  F.  E.  Fowle:  Volcanoes  and  Climate;  Smiths.  Misc. 
Coll.,  Vol.  60,  1913,  24  pp. 

W.  J.  Humphreys:  Volcanic  dust  and  other  factors  in  the  production  of 
climatic  and  their  possible  relation  to  ice  ages;  Bull.  Mount  Weather 
Observatory,  Vol.  6,  Part  1,  1913,  26  pp.  Also,  Physics  of  the  Air,  1920. 


46  CLIMATIC  CHANGES 

pyrheliometer.  This  summing  up  of  the  relations  between 
the  heat  received  from  the  sun,  and  the  occurrence  of 
explosive  volcanic  eruptions  leaves  little  room  for  doubt 
that  at  frequent  intervals  during  the  last  century  and  a 
half  a  slight  lowering  of  terrestrial  temperature  has 
actually  occurred  after  great  eruptions.  Nevertheless,  it 
does  not  justify  Humphreys'  final  conclusion  that  "phe- 
nomena within  the  earth  itself  suffice  to  modify  its  own 
climate,  .  .  .  that  these  and  these  alone  have  actually 
caused  great  changes  time  and  again  in  the  geologic 
past."  Humphreys  sees  so  clearly  the  importance  of  the 
purely  terrestrial  point  of  view  that  he  unconsciously 
slights  the  cosmic  standpoint  and  ignores  the  important 
solar  facts  which  he  himself  adduces  elsewhere  at  con- 
siderable length. 

In  addition  to  this  the  degree  to  which  the  temperature 
of  the  earth  as  a  whole  is  influenced  by  volcanic  eruptions 
is  by  no  means  so  clear  as  is  the  fact  that  there  is  some 
influence.  Arctowski,4  for  example,  has  prepared  numer- 
ous curves  showing  the  march  of  temperature  month 
after  month  for  many  years.  During  the  period  from 
1909  to  1913,  which  includes  the  great  eruption  of  Katmai 
in  Alaska,  low  temperature  is  found  to  have  prevailed  at 
the  time  of  the  eruption,  but,  as  Arctowski  puts  it,  on  the 
basis  of  the  curves  for  150  stations  in  all  parts  of  the 
world :  *  *  The  supposition  that  these  abnormally  low  tem- 
peratures were  due  to  the  veil  of  volcanic  dust  produced 
by  the  Katmai  eruption  of  June  6, 1912,  is  completely  out 
of  the  question.  If  that  had  been  the  case,  temperature 
would  have  decreased  from  that  date  on,  whereas  it  was 
decreasing  for  more  than  a  year  before  that  date." 

*  H.  Arctowski:  The  Pleionian  Cycle  of  Climatic  Fluctuations;  Am. 
Jour.  Sci.,  Vol.  42,  1916,  pp.  27-33.  See  also  Annals  of  the  New  York 
Academy  of  Sciences,  Vol.  24,  1914. 


HYPOTHESES  OF  CLIMATIC  CHANGES          47 

Koppen,5  in  his  comprehensive  study  of  temperature 
for  a  hundred  years,  also  presents  a  strong  argument 
against  the  idea  that  volcanic  eruptions  have  an  im- 
portant place  in  determining  the  present  temperature  of 
the  earth.  A  volcanic  eruption  is  a  sudden  occurrence. 
Whatever  effect  is  produced  by  dust  thrown  into  the  air 
must  occur  within  a  few  months,  or  as  soon  as  the  dust 
has  had  an  opportunity  to  be  wafted  to  the  region  in 
question.  When  the  dust  arrives,  there  will  be  a  rapid 
drop  through  the  few  degrees  of  temperature  which  the 
dust  is  supposed  to  be  able  to  account  for,  and  thereafter 
a  slow  rise  of  temperature.  If  volcanic  eruptions  actually 
caused  a  frequent  lowering  of  terrestrial  temperature  in 
the  hundred  years  studied  by  Koppen,  there  should  be 
more  cases  where  the  annual  temperature  is  decidedly 
below  the  normal  than  where  it  shows  a  large  departure 
in  the  opposite  direction.  The  contrary  is  actually  the 
case. 

A  still  more  important  argument  is  the  fact  that  the 
earth  is  now  in  an  intermediate  condition  of  climate. 
Throughout  most  of  geologic  time,  as  we  shall  see  again 
and  again,  the  climate  of  the  earth  has  been  milder  than 
now.  Regions  like  Greenland  have  not  been  the  seat  of 
glaciers,  but  have  been  the  home  of  types  of  plants  which 
now  thrive  in  relatively  low  latitudes.  In  other  words,  the 
earth  is  today  only  part  way  from  a  glacial  epoch  to  what 
may  be  called  the  normal,  mild  climate  of  the  earth — a 
climate  in  which  the  contrast  from  zone  to  zone  was  much 
less  than  now,  and  the  lower  air  averaged  warmer.  Hence 
it  seems  impossible  to  avoid  the  conclusion  that  the 
cause  of  glaciation  is  still  operating  with  considerable 

eW.  Koppen:  tfber  mehrjahrige  Perioden  der  Witterung  ins  besondere 
iizer  die  II-jahrige  Periode  der  Temperatur.  Also,  Lufttemperaturen 
Sonnenflecke  und  Vulcanausbriiche ;  Meteorologische  Zeitschrift,  Vol.  7, 
1914,  pp.  305-328. 


48  CLIMATIC  CHANGES 

although  diminished  efficiency.  But  volcanic  dust  is 
obviously  not  operating  to  any  appreciable  extent  at 
present,  for  the  upper  air  is  almost  free  from  dust  a  large 
part  of  the  time. 

Again,  as  Chamberlin  suggests,  let  it  be  supposed  that 
a  Krakatoan  eruption  every  two  years  would  produce  a 
glacial  period.  Unless  the  most  experienced  field  workers 
on  the  glacial  formations  are  quite  in  error,  the  various 
glacial  epochs  of  the  Pleistocene  glacial  period  had  a 
joint  duration  of  at  least  150,000  years  and  perhaps  twice 
as  much.  That  would  require  75,000  Krakatoan  eruptions. 
But  where  are  the  pits  and  cones  of  such  eruptions? 
There  has  not  been  time  to  erode  them  away  since  the 
Pleistocene  glaciation.  Their  beds  of  volcanic  ash  would 
presumably  be  as  voluminous  as  the  glacial  beds,  but 
there  do  not  seem  to  be  accumulations  of  any  such  size. 
Even  though  the  same  volcano  suffered  repeated  explo- 
sions, it  seems  impossible  to  find  sufficient  fresh  volcanic 
debris.  Moreover,  the  volcanic  hypothesis  has  not  yet 
offered  any  mechanism  for  systematic  glacial  variations. 
Hence,  while  the  hypothesis  is  important,  we  must  search 
further  for  the  full  explanation  of  glacial  fluctuations, 
historic  pulsations,  and  the  earth's  present  quasi-glacial 
climate. 

V.  The  Hypothesis  of  Polar  Wandering.  Another  hy- 
pothesis, which  has  some  adherents,  especially  among 
geologists,  holds  that  the  position  of  the  earth 's  axis  has 
shifted  repeatedly  during  geological  times,  thus  causing 
glaciation  in  regions  which  are  not  now  polar.  Astrophys- 
icists, however,  are  quite  sure  that  no  agency  could 
radically  change  the  relation  between  the  earth  and  its 
axis  without  likewise  altering  the  orbits  of  the  planets  to 
a  degree  that  would  be  easily  recognized.  Moreover,  the 
distribution  of  the  centers  of  glaciation  both  in  the  Per- 


HYPOTHESES  OF  CLIMATIC  CHANGES         49 

mian  and  Pleistocene  periods  does  not  seem  to  conform 
to  this  hypothesis. 

VI.  The  Thermal  Solar  Hypothesis.  The  only  other 
explanations  of  the  climatic  changes  of  glacial  and  his- 
toric times  which  now  seem  to  have  much  standing  are 
two  distinct  and  almost  antagonistic  solar  hypotheses. 
One  is  the  idea  that  changes  in  the  earth's  climate  are 
due  to  variations  in  the  heat  emitted  by  the  sun  and 
hence  in  the  temperature  of  the  earth.  The  other  is  the 
entirely  different  idea  that  climatic  changes  arise  from 
solar  conditions  which  cause  a  redistribution  of  the 
earth's  atmospheric  pressure  and  hence  produce  changes 
in  winds,  ocean  currents,  and  especially  storms.  This 
second,  or  "cyclonic,"  hypothesis  is  the  subject  of  a  book 
entitled  Earth  and  Sun,  which  is  to  be  published  as  a 
companion  to  the  present  volume.  It  will  be  outlined  in 
the  next  chapter.  The  other,  or  thermal,  hypothesis  may 
be  dismissed  briefly.  Unquestionably  a  permanent  change 
in  the  amount  of  heat  emitted  by  the  sun  would  perma- 
nently alter  the  earth's  climate.  There  is  absolutely  no 
evidence,  however,  of  any  such  change  during  geologic 
time.  The  evidence  as  to  the  earth's  cosmic  uniformity 
and  as  to  secular  progression  is  all  against  it.  Suppose 
that  for  thirty  or  forty  thousand  years  the  sun  cooled  off 
enough  so  that  the  earth  was  as  cool  as  during  a  glacial 
epoch.  As  glaciation  is  soon  succeeded  by  a  mild  climate, 
some  agency  would  then  be  needed  to  raise  the  sun's 
temperature.  The  impact  of  a  shower  of  meteorites  might 
accomplish  this,  but  that  would  mean  a  very  sudden  heat- 
ing, such  as  there  is  no  evidence  of  in  geological  history. 
In  fact,  there  is  far  more  evidence  of  sudden  cooling  than 
of  sudden  heating.  Moreover,  it  is  far  beyond  the  bounds 
of  probability  that  such  an  impact  should  be  repeated 
again  and  again  with  just  such  force  as  to  bring  the  cli- 


50  CLIMATIC  CHANGES 

mate  back  almost  to  where  it  started  and  yet  to  allow  for 
the  slight  changes  which  cause  secular  progression. 
Another  and  equally  cogent  objection  to  the  thermal  form 
of  solar  hypothesis  is  stated  by  Humphreys  as  follows : 
"A  change  of  the  solar  constant  obviously  alters  all  sur- 
face temperatures  by  a  roughly  constant  percentage. 
Hence  a  decrease  of  the  heat  from  the  sun  would  in  gen- 
eral cause  a  decrease  of  the  interzonal  temperature 
gradients;  and  this  in  turn  a  less  vigorous  atmospheric 
circulation,  and  a  less  copious  rain  or  snowfall — exactly 
the  reverse  of  the  condition,  namely,  abundant  precipita- 
tion, most  favorable  to  extensive  glaciation. ' ' 

This  brings  us  to  the  end  of  the  main  hypotheses  as  to 
climatic  changes,  aside  from  the  solar  cyclonic  hypothesis 
which  will  be  discussed  in  the  next  chapter.  It  appears 
that  variations  in  the  position  of  the  earth  at  perihelion 
have  a  real  though  slight  influence  in  causing  cycles  with 
a  length  of  about  2^000  years.  Changes  in  the  carbon 
dioxide  of  the  air  probably  have  a  more  important  but 
extremely  slow  influence  upon  geologic  oscillations. 
Variations  in  the  size,  shape,  and  height  of  the  continents 
are  constantly  causing  all  manner  of  climatic  complica- 
tions, but  do  not  cause  rapid  fluctuations  and  pulsations. 
The  eruption  of  volcanic  dust  appears  occasionally  to 
lower  the  temperature,  but  its  potency  to  explain  the 
complex  climatic  changes  recorded  in  the  rocks  has  prob- 
ably been  exaggerated.  Finally,  although  minor  changes 
in  the  amount  of  heat  given  out  by  the  sun  occur  con- 
stantly and  have  been  demonstrated  to  have  a  climatic 
effect,  there  is  jog  evidence  that  such  changes  are  the  main 
cause  of  the  climatic  phenomena  which  we  are  trying  to 
explain.  Nevertheless,  in  connection  with  other  solar 
changes  they  may  be  of  high  importance. 


CHAPTER  IV 
THE  SOLAR  CYCLONIC  HYPOTHESIS 

THE  progress  of  science  is  made  up  of  a  vast  suc- 
cession of  hypotheses.  The  majority  die  in  early 
infancy.  A  few  live  and  are  for  a  time  widely 
accepted.  Then  some  new  hypothesis  either  destroys  them 
completely  or  shows  that,  while  they  contain  elements  of 
truth,  they  are  not  the  whole  truth.  In  the  previous  chap- 
ter we  have  discussed  a  group  of  hypotheses  of  this  kind, 
and  have  tried  to  point  out  fairly  their  degree  of  truth  so 
far  as  it  can  yet  be  determined.  In  this  chapter  we  shall 
outline  still  another  hypothesis,  the  relation  of  which  to  ^ 
present  climatic  conditions  has  been  fully  developed  in  v 
Earth  and  Sun;  while  its  relation  to  the  past  will  be  ex- 
plained in  the  present  volume.  This  hypothesis  is  not 
supposed  to  supersede  the  others,  for  so  far  as  they  are 
true  they  cannot  be  superseded.  It  merely  seems  to  ex- 
plain some  of  the  many  conditions  which  the  other 
hypotheses  apparently  fail  to  explain.  To  suppose  that 
it  will  suffer  a  fate  more  glorious  than  its  predecessors 
would  be  presumptuous.  The  best  that  can  be  hoped  is 
that  after  it  has  been  pruned,  enriched,  and  modified,  it 
may  take  its  place  among  the  steps  which  finally  lead  to 
the  goal  of  truth. 

In  this  chapter  the  new  hypothesis  will  be  sketched  in 
broad  outline  in  order  that  in  the  rest  of  this  book  the 
reader  may  appreciate  the  bearing  of  all  that  is  said. 
Details  of  proof  and  methods  of  work  will  be  omitted, 


52  CLIMATIC  CHANGES 

\j2  since  they  are  given  in  Earth  and  Sun.  For  the  sake  of 
brevity  and  clearness  the  main  conclusions  will  be  stated 
without  the  qualifications  and  exceptions  which  are  fully 
explained  in  that  volume.  Here  it  will  be  necessary  to 
pass  quickly  over  points  which  depart  radically  from  ac- 
cepted ideas,  and  which  therefore  must  arouse  serious 
question  in  the  minds  of  thoughtful  readers.  That,  how- 
ever, is  a  necessary  consequence  of  the  attempt  which 
this  book  makes  to  put  the  problem  of  climate  in  such 
form  that  the  argument  can  be  followed  by  thoughtful 
students  in  any  branch  of  knowledge  and  not  merely  by 
specialists.  Therefore,  the  specialist  can  merely  be  asked 
to  withhold  judgment  until  he  has  read  all  the  evidence 
as  given  in  Earth  and  Sun,  and  then  to  condemn  only 
those  parts  that  are  wrong  and  not  the  whole  argument. 
Without  further  explanation  let  us  turn  to  our  main 
problem.  In  the  realm  of  climatology  the  most  important 
discovery  of  the  last  generation  is  that  variations  in  the 
weather  depend  on  variations  in  the  activity  of  the  sun's 
atmosphere.  The  work  of  the  great  astronomer,  New- 
comb,  and  that  of  the  great  climatologist,  Koppen,  have 
shown  beyond  question  that  the  temperature  of  the 
earth's  surface  varies  in  harmony  with  variations  in  the 
number  and  area  of  sunspots.1  The  work  of  Abbot  has 
shown  that  the  amount  of  heat  radiated  from  the  sun  also 
varies,  and  that  in  general  the  variations  correspond  with 
those  of  the  sunspots,  although  there  are  exceptions, 
especially  when  the  spots  are  fewest.  Here,  however, 
there  at  once  arises  a  puzzling  paradox.  The  earth  cer- 

i  The  so-called  sunspot  numbers  to  which  reference  is  made  again  and 
again  in  this  book  are  based  on  a  system  devised  by  Wolf  and  revised  by 
A.  Wolfer.  The  number  and  size  of  the  spots  are  both  taken  into  account. 
The  numbers  from  1749  to  1900  may  be  found  in  the  Monthly  Weather 
Eeview  for  April,  1902,  and  from  1901  to  1918  in  the  same  journal  for 
1920. 


THE  SOLAR  CYCLONIC  HYPOTHESIS  53 

tainly  owes  its  warmth  to  the  sun.  Yet  when  the  sun  emits 
the  most  energy,  that  is,  when  sunspots  are  most  numer- 
ous, the  earth's  surface  is  coolest.  Doubtless  the  earth 
receives  more  heat  than  usual  at  such  times,  and  the 
upper  air  may  be  warmer  than  usual.  Here  we  refer  only 
to  the  air  at  the  earth's  surface. 

Another  large  group  of  investigators  havejshown  that 
atmospheric  pressure  also  varies  in  harmony  with  the  * 
number  of  sunspots.  Some  parts  of  the  earth's  surface 
have  one  kind  of  variation  at  times  of  many  sunspots  and 
other  parts  the  reverse.  These  differences  are  systematic 
and  depend  largely  on  whether  the  region  in  question 
happens  to  have  high  atmospheric  pressure  or  low.  The 
net  ^esult  is  that  when  sunspots  are  numerous  the 
earth's  storminess  increases,  and  the  atmosphere  is 
thrown  into  commotion.  This  interferes  with  the  stable 
planetary  winds,  such  as  the  trades  of  low  latitudes  and 
the  prevailing  westerlies  of  higher  latitudes.  Instead  of 
these  regular  winds  and  the  fair  weather  which  they 
bring,  there  is  a  tendency  toward  frequent  tropical  hurri- 
canes in  the  lower  latitudes  and  toward  more  frequent 
"  and  severe  storms  of  the  ordinary  type  in  the  latitudes 
where  the  world's  most  progressive  nations  now  live. 
With  the  change  in  storminess  there  naturally  goes  a 
change  in  rainfall.  Not  all  parts  of  the  world,  however,^ 
have  increased  storminess  and  more  abundant  rainfall  ' 
when  sunspots  are  numerous.  Some  parts  change  in  the 
opposite  way.  Thus  when  the  sun's  atmosphere  is  par- 
ticularly disturbed,  the  contrasts  between  different  parts 
of  the  earth's  surface  are  increased.  For  example,  the 
northern  United  States  and  southern  Canada  become 
more  stormy  and  rainy,  as  appears  in  Fig.  2,  and  the 
same  is  true  of  the  Southwest  and  along  the  south  Atlan- 
tic coast.  In  a  crescent-shaped  central  area,  however, 


54  CLIMATIC  CHANGES 

extending  from  Wyoming  through  Missouri  to  Nova 
Scotia,  the  number  of  storms  and  the  amount  of  rainfall 
decrease. 

The  two  controlling  factors  of  any  climate  are  the 
temperature  and  the  atmospheric  pressure,  for  they  de- 
termine the  winds,  the  storms,  and  thus  the  rainfall.  A 
study  of  the  temperature  seems  to  show  that  the  peculiar 
paradox  of  a  hot  sun  and  a  cool  earth  is  due  largely  to 
the  increased  storminess  during  times  of  many  sunspots. 
The  earth's  surface  is  heated  by  the  rays  of  the  sun,  but 


Fig.  2.  Storminess  at  sunspot  maxima  vs.  minima. 

(After  Kullmer.) 

Based  on  nine  years'  nearest  sunspot  minima  and  nine  years'  nearest  sun- 
spot  maxima  in  the  three  sunspot  cycles  from  1888  to  1918.  Heavy  shading 
indicates  excess  of  storminess  when  sunspots  are  numerous.  Figures  indicate 
average  yearly  number  of  storms  by  which  years  of  maximum  sunspots 
exceed  those  of  minimum  sunspots. 


THE  SOLAR  CYCLONIC  HYPOTHESIS  55 

most  of  the  rays  do  not  in  themselves  heat  the  air  as 
they  pass  through  it.  The  air  gets  its  heat  largely  from 
the  heat  absorbed  by  the  water  vapor  which  is  intimately 
mingled  with  its  lower  portions,  or  from  the  long  heat 
waves  sent  out  by  the  earth  after  it  has  been  warmed  by 
the  sun.  The  faster  the  air  moves  along  the  earth's  sur- 
face the  less  it  becomes  heated,  and  the  more  heat  it  takes 
away.  This  sounds  like  a  contradiction,  but  not  to  anyone 
who  has  tried  to  heat  a  stove  in  the  open  air.  If  the  air 
is  still,  the  stove  rapidly  becomes  warm  and  so  does  the 
air  around  it.  If  the  wind  is  blowing,  the  cool  air  delays 
the  heating  of  the  stove  and  prevents  the  surface  from  tfP4 
ever  becoming  as  hot  as  it  would  otherwise.  That  seems 
to  be  what  happens  on  a  large  scale  when  sunspots  are 
numerous.  The  sun  actually  sends  to  the  earth  more 
energy  than  usual,  but  the  air  moves  with  such  unusual 
rapidity  that  it  actually  cools  the  earth's  surface  a  trifle 
by  carrying  the  extra  heat  to  high  levels  where  it  is  lost 
into  space. 

There  has  been  much  discussion  as  to  why  storms  are 
numerous  when  the  sun's  atmosphere  is  disturbed.  Many 
investigators   have   supposed  it  was   due  entirely  and 
directly  to  the  heating  of  the  earth's  surface  by  the  sun. 
This,  however,  needs  modification  for  several  reasons. 
In  the  first  place,  recent  investigations  show  that  in  a| 
great  many  cases  changes  in  barometric  pressure  precede 
changes  in  temperature  and  apparently  cause  them  by 
altering  the  winds  and  producing  storms.  This  is  the! 
opposite  of  what  would  happen  if  the  effect  of  solar  heat ' 
upon  the  earth's  surface  were  the  only  agency.  In  the 
second  place,  if  storms  were  due  exclusively  to  variations 
in  the  ordinary  solar  radiation  which  comes  to  the  earth 
as  light  and  is  converted  into  heat,  the  solar  effect  ought 


56  CLIMATIC  CHANGES 

to  be  most  pronounced  when  the  center  of  the  sun's 
visible  disk  is  most  disturbed.  As  a  matter  of  fact  the 
storminess  is  notably  greatest  when  the  edges  of  the 
solar  disk  are  most  disturbed.  These  facts  and  others  lead 
to  the  conclusion  that  some  agency  other  than  heat  must 
also  play  some  part  in  producing  storminess. 

The  search  for  this  auxiliary  agency  raises  many  diffi- 
cult questions  which  cannot  yet  be  answered.  On  the 
whole  the  weight  of  evidence  suggests  that  electrical 
phenomena  of  some  kind  are  involved,  although  varia- 
tions in  the  amount  of  ultra-violet  light  may  also  be 
important.  Many  investigators  have  shown  that  the  sun 
emits  electrons.  Hale  has  proved  that  the  sun,  like  the 
earth,  is  magnetized.  Sunspots  also  have  magnetic  fields 
the  strength  of  which  is  often  fifty  times  as  great  as  that 
of  the  sun  as  a  whole.  If  electrons  are  sent  to  the  earth, 
they  must  move  in  curved  paths,  for  they  are  deflected 
by  the  sun's  magnetic  field  and  again  by  the  earth's 
magnetic  field.  The  solar  deflection  may  cause  their 
effects  to  be  greatest  when  the  spots  are  near  the  sun's 
margin;  the  terrestrial  deflection  may  cause  concentra- 
tion in  bands  roughly  concentric  with  the  magnetic  poles 
of  the  earth.  These  conditions  correspond  with  the  known 
facts. 

Farther  than  this  we  cannot  yet  go.  The  calculations  of 
Humphreys  seem  to  indicate  that  the  direct  electrical 
effect  of  the  sun's  electrons  upon  atmospheric  pressure 
is  too  small  to  be  of  appreciable  significance  in  intensify- 
ing storms.  On  the  other  hand  the  peculiar  way  in  which 
activity  upon  the  margins  of  the  sun  appears  to  be  corre- 
lated not  only  with  atmospheric!  electricity,  but  with 
barometric  pressure,  seems  to  be  equally  strong  evidence 
in  the  other  direction.  Possibly  the  sun's  electrons  and 
its  electrical  waves  produce  indirect  effects  by  being 


(Pi 

THE  SOLAR  CYCLONIC  HYPOTHESIS  57 

converted  into  heat,  or  by  causing  the  formation  of  ozone 
and  the  condensation  of  water  vapor  in  the  upper "  air. 
Any  one  of  these  processes  would  raise  the  temperature 
of  the  upper  air,  for  the  ozone  and  the  water  vapor  would 
be  formed  there  and  would  tend  to  act  as  a  blanket  to 
hold  in  the  earth's  heat.  But  any  such  change  in  the  tem- 
perature of  the  upper  air  would  influence  the  lower  air 
through  changes  in  barometric  pressure.  These  con- 
siderations are  given  here  because  the  thoughtful  reader 
is  likely  to  inquire  how  solar  activity  can  influence 
storminess.  Moreover,  at  the  end  of  this  book  we  shall 
take  up  certain  speculative  questions  in  which  an  elec- 
trical hypothesis  will  be  employed.  For  the  main  por- 
tions of  this  book  it  makes  no  difference  how  the  sun's 
variations  influence  the  earth's  atmosphere.  The  only 
essential  point  is  that  when  the  solar  atmosphere  is  active 
the  storminess  of  the  earth  increases,  and  that  is  a  matter 
of  direct  observation. 

Let  us  now  inquire  into  the  relation  between  the  small 
cyclonic  vacillations  of  the  weather  and  the  types  of 
climatic  changes  known  as  historic  pulsations  and  glacial 
fluctuations.  One  of  the  most  interesting  results  of  recent 
investigations  is  the  evidence  that  sunspot  cycles  on  a 
small  scale  present  almost  the  same  phenomena  as  do 
historic  pulsations  and  glacial  fluctuations.  For  instance, 
when  sunspots  are  numerous,  storminess  increases 
markedly  in  a  belt  near  the  northern  border  of  the  area 
of  greatest  storminess,  that  is,  in  southern  Canada  and 
thence  across  the  Atlantic  to  the  North  Sea  and  Scandi- 
navia. (See  Figs.  2  and  3.)  Corresponding  with  this  is  the 
fact  that  the  evidence  as  to  climatic  pulsations  in  historic 
times  indicates  that  regions  along  this  path,  for  instance 
Greenland,  the  North  Sea  region,  and  southern  Scandi- 


Fig.  3.  Relative  rainfall  at  times  of  increasing  and  decreasing 

sunspots. 

Heavy  shading,  more  rain  with  increasing  spots.  Light  shading,  more  rain  with  de- 
creasing spots.  No  data  for  unshaded  areas. 

Figures  indicate  percentages  of  the  average  rainfall  by  which  the  rainfall  during 
periods  of  increasing  spots  exceeds  or  falls  short  of  rainfall  during  periods  of  decreas- 
ing spots.  The  excess  or  deficiency  is  stated  in  percentages  of  the  average.  Eainfall 
data  from  Walker:  Sunspots  and  Rainfall. 


Fig.  3.  Relative  rainfall  at  times  of  increasing  and  decreasing 

sunspots. 

Heavy  shading,  more  rain  with  increasing  spots.  Light  shading,  more  rain  with  de- 
creasing spots.  No  data  for  unshaded  areas. 

Figures  indicate  percentages  of  the  average  rainfall  by  which  the  rainfall  during 
periods  of  increasing  spots  exceeds  or  falls  short  of  rainfall  during  periods  of  decreas- 
ing spots.  The  excess  or  deficiency  is  stated  in  percentages  of  the  average.  Kainfall 
data  from  Walker :  Sunspots  and  Eainf all. 


60  CLIMATIC  CHANGES 

navia,  were  visited  by  especially  frequent  and  severe 
storms  at  the  climax  of  each  pulsation.  Moreover,  the 
greatest  accumulations  of  ice  in  the  glacial  period  were 
on  the  poleward  border  of  the  general  regions  where  now 
the  storms  appear  to  increase  most  at  times  of  solar 
activity. 

Even  more  clear  is  the  evidence  from  other  regions 
where  storms  increase  at  times  of  many  sunspots.  One 
such  region  includes  the  southwestern  United  States, 
while  another  is  the  Mediterranean  region  and  the  semi- 
arid  or  desert  parts  of  Asia  farther  east.  In  these  regions 
innumerable  ruins  and  other  lines  of  evidence  show  that 
at  the  climax  of  each  climatic  pulsation  there  was  more 
storminess  and  rainfall  than  at  present,  just  as  there 
now  is  when  the  sun  is  most  active.  In  still  earlier  times, 
while  ice  was  accumulating  farther  north,  the  basins  of 
these  semi-arid  regions  were  filled  with  lakes  whose 
strands  still  remain  to  tell  the  tale  of  much-increased 
rainfall  and  presumable  storminess.  If  we  go  back  still 
further  in  geological  times  to  the  Permian  glaciation,  the 
areas  where  ice  accumulated  most  abundantly  appear  to 
be  the  regions  where  tropical  hurricanes  produce  the 
greatest  rainfall  and  the  greatest  lowering  of  tempera- 
ture at  times  of  many  sunspots.  From  these  and  many 
other  lines  of  evidence  it  seems  probable  that  historic 
pulsations  and  glacial  fluctuations  are  nothing  more  than 
sunspot  cycles  on  a  large  scale.  It  is  one  of  the  funda- 
mental rules  of  science  to  reason  from  the  known  to  the 
unknown,  from  the  near  to  the  far,  from  the  present  to 
the  past.  Hence  it  seems  advisable  to  investigate  whether 
any  of  the  climatic  phenomena  of  the  past  may  have 
arisen  from  an  intensification  of  the  solar  conditions 
which  now  appear  to  give  rise  to  similar  phenomena  on 
a  small  scale. 


THE  SOLAR  CYCLONIC  HYPOTHESIS  61 

The  rest  of  this  chapter  will  be  devoted  to  a  resume 
of  certain  tentative  conclusions  which  have  no  bearing 
on  the  main  part  of  this  book,  but  which  apply  to  the 
closing  chapters.  There  we  shall  inquire  into  the  perio- 
dicity of  the  climatic  phenomena  of  geological  times,  and 
shall  ask  whether  there  is  any  reason  to  suppose  that  the 
sun's  activity  has  exhibited  similar  periodicity.  This 
leads  to  an  investigation  of  the  possible  causes  of  dis- 
turbances in  the  sun's  atmosphere.  It  is  generally  as- 
sumed that  sunspots,  solar  prominences,  the  bright  clouds 
known  as  faculse,  and  other  phenomena  denoting  a  per- 
turbed state  of  the  solar  atmosphere,  are  due  to  some 
cause  within  the  sun.  Yet  the  limitation  of  these  phe- 
nomena, especially  the  sunspots,  to  restricted  latitudes, 
as  has  been  shown  in  Earth  and  Sun,  does  not  seem  to  be 
in  harmony  with  an  internal  solar  origin,  even  though 
a  banded  arrangement  may  be  normal  for  a  rotating 
globe.  The  fairly  regular  periodicity  of  the  sunspots  ) 
seems  equally  out  of  harmony  with  an  internal  origii 
Again,  the  solar  atmosphere  has  two  kinds  of  circula- 
tion, one  the  so-called  "rice  grains,"  and  the  other 
the  spots  and  their  attendant  phenomena.  Now  the  rice 
grains  present  the  appearance  that  would  be  expected  in 
an  atmospheric  circulation  arising  from  the  loss  of  heat 
by  the  outer  part  of  a  gaseous  body  like  the  sun.  For 
these  reasons  and  others,  numerous  good  thinkers  from 
Wolf  to  Schuster  have  held  that  sunspots  owe  their 
periodicity  to  causes  outside  the  sun.  The  only  possible 
cause  seems  to  be  the  planets,  acting  either  through 
gravitation,  through  forces  of  an  electrical  origin,  or 
through  some  other  agency.  Various  new  investigations 
which  are  described  in  Earth  and  Sun  support  this  con- 
clusion. The  chief  difficulty  in  accepting  it  hitherto  has 
been  that  although  Jupiter,  because  of  its  size,  would  be 


62  CLIMATIC  CHANGES 

expected  to  dominate  the  sunspot  cycle,  its  period  of 
11.86  years  has  not  been  detected.  The  sunspot  cycle  has 
appeared  to  average  11.2  years  in  length,  and  has  been 
called  the  11-year  cycle.  Nevertheless,  a  new  analysis  of 
the  sunspot  data  shows  that  when  attention  is  concen- 
trated upon  the  major  maxima,  which  are  least  subject  to 
retardation  or  acceleration  by  other  causes,  a  periodicity 
closely  approaching  that  of  Jupiter  is  evident.  Moreover, 


when  the  effects  of  Jupiter,  Saturn,  and  the  other  planets 
are  combined,  they  produce  a  highly  variable  curve  which 
has  an  extraordinary  resemblance  to  the  sunspot  curve. 
The  method  by  which  the  planets  influence  the  sun's 
atmosphere  is  still  open  to  question.  It  may  be  through 
tides,  through  the  direct  effect  of  gravitation,  through 
electro-magnetic  forces,  or  in  some  other  way.  Whichever 
it  may  be,  the  result  may  perhaps  be  slight  differences  of 
atmospheric  pressure  upon  the  sun.  Such  differences 
may  set  in  motion  slight  whirling  movements  analogous 
to  terrestrial  storms,  and  these  presumably  gather  mo- 
mentum from  the  sun's  own  energy.  Since  the  planet- 
ary influences  vary  in  strength  because  of  the  continuous 
change  in  the  rela^ve^dj-staiices  and  positions  of  the 
planets,  the  sun-s  atmosphere^appears  to  be  swayed  by 
cyclonic  disturbances  of  varying  degrees  of  severity.  The 
cyclonic  disturbances  known  as  sunspots  have  been 
proved  by  Hale  to  become  more  highly  electrified  as  they 
increase  in  intensity.  At  the  same  time  hot  gases  pre- 
sumably well  up  from  the  lower  parts  of  the  solar  atmos- 
phere and  thereby  cause  the  sun  to  emit  more  heat.  Thus 
by  one  means  or  another,  the  earth's  atmosphere  appears 
to  be  set  in  commotion  and  cycles  of  climate  are  in- 
augurated. 

If  the  preceding  reasoning  is  correct,  any  disturbance 
of  the  solar  atmosphere  must  have  an  effect  upon  the 


THE  SOLAR  CYCLONIC  HYPOTHESIS  63 

earth's  climate.  If  the  disturbance  were  great  enough  and 
of  the  right  nature  it  might  produce  a  glacial  epoch.  The 
planets  are  by  no  means  the  only  bodies  which  act  upon 
the  sun,  for  that  body  sustains  a  constantly  changing 
relation  to  millions  of  other  celestial  bodies  of  all  sizes 
up  to  vast  universes,  and  at  all  sorts  of  distances.  If  the 
sun  and  another  star  should  approach  near  enough  to  one 
another,  it  is  certain  that  the  solar  atmosphere  would  be 
disturbed  much  more  than  at  present. 

Here  we  must  leave  the  cyclonic  hypothesis  of  climate 
and  must  refer  the  reader  once  more  to  Earth  and  Sun  \  J 
for  fuller  details.  In  the  rest  of  this  book  we  shall  discuss 
the  nature  of  the  climatic  changes  of  past  times  and  shall 
inquire  into  their  relation  to  the  various  climatic  hypothe- 
ses mentioned  in  the  last  two  chapters.  Then  we  shall 
inquire  into  the  possibility  that  the  solar  system  has  ever 
been  near  enough  to  any  of  the  stars  to  cause  appreciable 
disturbances  of  the  solar  atmosphere.  We  shall  complete 
our  study  by  investigating  the  vexed  question  of  why 
movements  of  the  earth's  crust,  such  as  the  uplifting  of 
continents  and  mountain  chains,  have  generally  occurred 
at  the  same  time  as  great  climatic  fluctuations.  This 
would  not  be  so  surprising  were  it  not  that  the  climatic 
phenomena  appear  to  have  consisted  of  highly  complex 
cycles  while  the  uplift  has  been  a  relatively  steady  move- 
ment in  one  direction.  We  shall  find  some  evidence  that 
the  solar  disturbances  which  seem  to  cause  climati<£ 
changes  also  have  a  relation  to  movements  of  the  crust.  !'/',/ 


CHAPTER  V 
THE  CLIMATE  OF  HISTORY1 

E  are  now  prepared  to  consider  the  climate  of 
the  past.  The  first  period  to  claim  attention  is 
the  few  thousand  years  covered  by  written 
history.  Strangely  enough,  the  conditions  during  this 
time  are  known  with  less  accuracy  than  are  those  of 
geological  periods  hundreds  of  times  more  remote.  Yet 
if  pronounced  changes  have  occurred  since  the  days  of 
the  ancient  Babylonians  and  since  the  last  of  the  post- 
glacial stages,  they  are  of  great  importance  not  only 
because  of  their  possible  historic  effects,  but  because  they 
bridge  the  gap  between  the  little  variations  of  climate 
which  are  observable  during  a  single  lifetime  and  the 
great  changes  known  as  glacial  epochs.  Only  by  bridging 
the  gap  can  we  determine  whether  there  is  any  genetic 
relation  between  the  great  changes  and  the  small.  A  full 
discussion  of  the  climate  of  historic  times  is  not  here 
advisable,  for  it  has  been  considered  in  detail  in  numer- 
ous other  publications.2  Our  most  profitable  course  would 
seem  to  be  to  consider  first  the  general  trend  of  opinion 
and  then  to  take  up  the  chief  objections  to  each  of  the 
main  hypotheses. 

In  the  hot  debate  over  this  problem  during  recent 

iMuch  of  this  chapter  is  taken  from  The  Solar  Hypothesis  of  Climatic 
Changes;  Bull.  Geol.  Soc.  Am.,  Vol.  25,  1914. 

2  Ellsworth  Huntington :  Explorations  in  Turkestan,  1905 ;  The  Pulse  of 
Asia,  1907;  Palestine  and  Its  Transformation,  1911  j  The  Climatic  Factor, 
1915;  World  Power  and  Evolution,  1919. 


THE  CLIMATE  OF  HISTORY  65 

decades  the  ideas  of  geographers  seem  to  have  gone 
through  much  the  same  metamorphosis  as  have  those  of 
geologists  in  regard  to  the  climate  of  far  earlier  times. 

As  every  geologist  well  knows,  at  the  dawn  of  geology 
people  believed  in  climatic  uniformity — that  is,  it  was 
supposed  that  since  the  completion  of  an  original  creative 
act  there  had  been  no  important  changes.  This  view 
quickly  disappeared  and  was  superseded  by  the  hypothe- 
sis of  progressive  cooling  and  drying,  an  hypothesis 
which  had  much  to  do  with  the  development  of  the  nebu- 
lar hypothesis,  and  which  has  in  turn  been  greatly 
strengthened  by  that  hypothesis.  The  discovery  of  evi- 
dence of  widespread  continental  glaciation,  however, 
necessitated  a  modification  of  this  view,  and  succeeding 
years  have  brought  to  light  a  constantly  increasing  num- 
ber of  glacial,  or  at  least  cool,  periods  distributed 
throughout  almost  the  whole  of  geological  time.  More- 
over, each  year,  almost,  brings  new  evidence  of  the  great 
complexity  of  glacial  periods,  epochs,  and  stages.  Thus, 
for  many  decades,  geologists  have  more  and  more  been 
led  to  believe  that  in  spite  of  surprising  uniformity,  when 
viewed  in  comparison  with  the  cosmic  possibilities,  the 
climate  of  the  past  has  been  highly  unstable  from  the 
viewpoint  of  organic  evolution,  and  its  changes  have  been 
of  all  degrees  of  intensity. 

Geographers  have  lately  been  debating  the  reality  of 
historic  changes  of  climate  in  the  same  way  in  which 
geologists  debated  the  reality  of  glacial  epochs  and 
stages.  Several  hypotheses  present  themselves  but  these 
may  all  be  grouped  under  three  headings;  namely,  the 
hypotheses  of  (1)  progressive  desiccation,  (2)  climatic 
uniformity,  and  (3)  pulsations.  The  hypothesis  of  pro- 
gressive desiccation  has  been  widely  advocated.  In  many 
of  the  drier  portions  of  the  world,  especially  between  30° 


66  CLIMATIC  CHANGES 

and  40°  from  the  equator,  and  preeminently  in  western 
and  central  Asia  and  in  the  southwestern  United  States, 
almost  innumerable  facts  seem  to  indicate  that  two  or 
three  thousand '  years  ago  the  climate  was  distinctly 
moister  than  at  present.  The  evidence  includes  old  lake 
strands,  the  traces  of  desiccated  springs,  roads  in  places 
now  too  dry  for  caravans,  other  roads  which  make  de- 
tours around  dry  lake  beds  where  no  lakes  now  exist,  and 
fragments  of  dead  forests  extending  over  hundreds  of 
square  miles  where  trees  cannot  now  grow  for  lack  of 
water.  Still  stronger  evidence  is  furnished  by  ancient 
ruins,  hundreds  of  which  are  located  in  places  which  are 
now  so  dry  that  only  the  merest  fraction  of  the  former 
inhabitants  could  find  water.  The  ruins  of  Palmyra,  in 
the  Syrian  Desert,  show  that  it  must  once  have  been  a 
city  like  modern  Damascus,  with  one  or  two  hundred 
thousand  inhabitants,  but  its  water  supply  now  suffices 
for  only  one  or  two  thousand.  All  attempts  to  increase  the 
water  supply  have  had  only  a  slight  effect  and  the  water 
is  notoriously  sulphurous,  whereas  in  the  former  days, 
when  it  was  abundant,  it  was  renowned  for  its  excellence. 
Hundreds  of  pages  might  be  devoted  to  describing  simi- 
lar ruins.  Some  of  them  are  even  more  remarkable  for 
their  dryness  than  is  Niya,  a  site  in  the  Tarim  Desert  of 
Chinese  Turkestan.  Yet  there  the  evidence  of  desiccation 
within  2000  years  is  so  strong  that  even  so  careful  and 
conservative  a  man  as  Hann,3  pronounces  it  "iiber- 
zeugend. ' ' 

A  single  quotation  from  scores  that  might  be  used  will 
illustrate  the  conclusions  of  some  of  the  most  careful 
archaeologists.4 

s  J.  Hann :  Klimatologie,  Vol.  1,  1908,  p.  352. 

*  H.  C.  Butler :  Desert  Syria,  the  Land  of  a  Lost  Civilization ;  Geographi- 
cal Review,  Feb.,  1920,  pp.  77-108. 


THE  CLIMATE  OF  HISTORY  67 

Among  the  regions  which  were  once  populous  and  highly 
civilized,  but  which  are  now  desert  and  deserted,  there  are  few 
which  were  more  closely  connected  with  the  beginnings  of  our 
own  civilization  than  the  desert  parts  of  Syria  and  northern 
Arabia.  It  is  only  of  recent  years  that  the  vast  extent  and  great 
importance  of  this  lost  civilization  has  been  fully  recognized  and 
that  attempts  have  been  made  to  reduce  the  extent  of  the  unex- 
plored area  and  to  discover  how  much  of  the  territory  which  has 
long  been  known  as  desert  was  formerly  habitable  and  inhabited. 
The  results  of  the  explorations  of  the  last  twenty  years  have  been 
most  astonishing  in  this  regard.  It  has  been  found  that  practi- 
cally all  of  the  wide  area  lying  between  the  coast  range  of  the 
eastern  Mediterranean  and  the  Euphrates,  appearing  upon  the 
maps  as  the  Syrian  Desert,  an  area  embracing  somewhat  more 
than  20,000  square  miles,  was  more  thickly  populated  than  any 
area  of  similar  dimensions  in  England  or  in  the  United  States 
is  today  if  one  excludes  the  immediate  vicinity  of  the  large 
modern  cities.  It  has  also  been  discovered  that  an  enormous 
desert  tract  lying  to  the  east  of  Palestine,  stretching  eastward 
and  southward  into  the  country  which  we  know  as  Arabia,  was 
also  a  densely  populated  country.  How  far  these  settled  regions 
extended  in  antiquity  is  still  unknown,  but  the  most  distant 
explorations  in  these  directions  have  failed  to  reach  the  end  of 
ruins  and  other  signs  of  former  occupation. 

The  traveler  who  has  crossed  the  settled,  and  more  or  less 
populous,  coast  range  of  northern  Syria  and  descended  into  the 
narrow  fertile  valley  of  the  Orontes,  encounters  in  any  farther 
journey  toward  the  east  an  irregular  range  of  limestone  hills 
lying  north  and  south  and  stretching  to  the  northeast  almost 
halfway  to  the  Euphrates.  These  hills  are  about  2,500  feet  high, 
rising  in  occasional  peaks  from  3,000  to  3,500  feet  above  sea  level. 
They  are  gray  and  unrelieved  by  any  visible  vegetation.  On 
ascending  into  the  hills  the  traveler  is  astonished  to  find  at  every 
turn  remnants  of  the  work  of  men's  hands,  paved  roads,  walls 
which  divided  fields,  terrace  walls  of  massive  structure.  Pres- 
ently he  comes  upon  a  small  deserted  and  partly  ruined  town 


68  CLIMATIC  CHANGES 

composed  of  buildings  large  and  small  constructed  of  beauti- 
fully wrought  blocks  of  limestone,  all  rising  out  of  the  barren 
rock  which  forms  the  ribs  of  the  hills.  If  he  mounts  an  eminence 
in  the  vicinity,  he  will  be  still  further  astonished  to  behold 
similar  ruins  lying  in  all  directions.  He  may  count  ten  or  fifteen 
or  twenty,  according  to  the  commanding  position  of  his  lookout. 
From  a  distance  it  is  often  difficult  to  believe  that  these  are  not 
inhabited  places;  but  closer  inspection  reveals  that  the  gentle 
hand  of  time  or  the  rude  touch  of  earthquake  has  been  laid  upon 
every  building.  Some  of  the  towns  are  better  preserved  than 
others;  some  buildings  are  quite  perfect  but  for  their  wooden 
roofs  which  time  has  removed,  others  stand  in  picturesque  ruins, 
while  others  still  are  level  with  the  ground.  On  a  far-off  hilltop 
stands  the  ruin  of  a  pagan  temple,  and  crowning  some  lofty  ridge 
lie  the  ruins  of  a  great  Christian  monastery.  Mile  after  mile  of 
this  barren  gray  country  may  be  traversed  without  encountering 
a  single  human  being.  Day  after  day  may  be  spent  in  traveling 
from  one  ruined  town  to  another  without  seeing  any  green 
thing  save  a  terebinth  tree  or  two  standing  among  the  ruins, 
which  have  sent  their  roots  down  into  earth  still  preserved  in 
the  foundations  of  some  ancient  building.  No  soil  is  visible 
anywhere  except  in  a  few  pockets  in  the  rock  from  which  it 
could  not  be  washed  by  the  torrential  rains  of  the  wet  season; 
yet  every  ruin  is  surrounded  with  the  remains  of  presses  for  the 
making  of  oil  and  wine.  Only  one  oasis  has  been  discovered  in 
these  high  plateaus. 

Passing  eastward  from  this  range  of  hills,  one  descends  into  a 
gently  rolling  country  that  stretches  miles  away  toward  the 
Euphrates.  At  the  eastern  foot  of  the  hills  one  finds  oneself  in  a 
totally  different  country,  at  first  quite  fertile  and  dotted  with 
frequent  villages  of  flat-roofed  houses.  Here  practically  all  the 
remains  of  ancient  times  have  been  destroyed  through  ages  of 
building  and  rebuilding.  Beyond  this  narrow  fertile  strip  the 
soil  grows  drier  and  more  barren,  until  presently  another  kind 
of  desert  is  reached,  an  undulating  waste  of  dead  soil.  Few  walls 
or  towers  or  arches  rise  to  break  the  monotony  of  the  unbroken 


THE  CLIMATE  OF  HISTORY  69 

landscape;  but  the  careful  explorer  will  find  on  closer  examina- 
tion that  this  region  was  more  thickly  populated  in  antiquity 
even  than  the  hill  country  to  the  west.  Every  unevenness  of  the 
surface  marks  the  site  of  a  town,  some  of  them  cities  of  con- 
siderable extent. 

We  may  draw  certain  very  definite  conclusions  as  to  the 
former  conditions  of  the  country  itself.  There  was  soil  upon  the 
northern  hills  where  none  now  exists,  for  the  buildings  now  show 
unfinished  foundation  courses  which  were  not  intended  to  be 
seen;  the  soil  in  depressions  without  outlets  is  deeper  than  it 
formerly  was;  there  are  hundreds  of  olive  and  wine  presses  in 
localities  where  no  tree  or  vine  could  now  find  footing;  and 
there  are  hillsides  with  ruined  terrace  walls  rising  one  above 
the  other  with  no  sign  of  earth  near  them.  There  was  also  a  large 
natural  water  supply.  In  the  north  as  well  as  in  the  south  we 
find  the  dry  beds  of  rivers,  streams,  and  brooks  with  sand  and 
pebbles  and  well-worn  rocks  but  no  water  in  them  from  one 
year's  end  to  the  other.  We  find  bridges  over  these  dry  streams 
and  crudely  made  washing  boards  along  their  banks  directly 
below  deserted  towns.  Many  of  the  bridges  span  the  beds  of 
streams  that  seldom  or  never  have  water  in  them  and  give 
clear  evidence  of  the  great  climatic  changes  that  have  taken 
place.  There  are  well  heads  and  well  houses,  and  inscriptions 
referring  to  springs;  but  neither  wells  nor  springs  exist  today 
except  in  the  rarest  instances.  Many  of  the  houses  had  their 
rock-hewn  cisterns,  never  large  enough  to  have  supplied  water 
for  more  than  a  brief  period,  and  corresponding  to  the  cisterns 
which  most  of  our  recent  forefathers  had  which  were  for  con- 
venience rather  than  for  dependence.  Some  of  the  towns  in 
southern  Syria  were  provided  with  large  public  reservoirs,  but 
these  are  not  large  enough  to  have  supplied  water  to  their 
original  populations.  The  high  plateaus  were  of  course  without 
irrigation;  but  there  are  no  signs,  even  in  the  lower  flatter 
country,  that  irrigation  was  ever  practiced;  and  canals  for  this 
purpose  could  not  have  completely  disappeared.  There  were 
forests  in  the  immediate  vicinity,  forests  producing  timbers  of 
great  length  and  thickness ;  for  in  the  north  and  northeast  prac- 


70  CLIMATIC  CHANGES 

tically  all  the  buildings  had  wooden  roofs,  wooden  intermediate 
floors,  and  other  features  of  wood.  Costly  buildings,  such  as 
temples  and  churches,  employed  large  wooden  beams ;  but  wood 
was  used  in  much  larger  quantities  in  private  dwellings,  shops, 
stables,  and  barns.  If  wood  had  not  been  plentiful  and  cheap — 
which  means  grown  near  by — the  builders  would  have  adopted 
the  building  methods  of  their  neighbors  in  the  south,  who  used 
very  little  wood  and  developed  the  most  perfect  type  of  lithic 
architecture  the  world  has  ever  seen.  And  here  there  exists  a 
strange  anomaly:  Northern  Syria,  where  so  much  wood  was 
employed  in  antiquity,  is  absolutely  treeless  now;  while  in  the 
mountains  of  southern  Syria,  where  wood  must  have  been 
scarce  in  antiquity  to  have  forced  upon  the  inhabitants  an  almost 
exclusive  use  of  stone,  there  are  still  groves  of  scrub  oak  and 
pine,  and  travelers  of  half  a  century  ago  reported  large  forests 
of  chestnut  trees.5  It  is  perfectly  apparent  that  large  parts  of 
Syria  once  had  soil  and  forests  and  springs  and  rivers,  while  it 
has  none  of  these  now,  and  that  it  had  a  much  larger  and  better 
distributed  rainfall  in  ancient  times  than  it  has  now. 

Professor  Butler's  careful  work  is  especially  interest- 
ing because  of  its  contrast  to  the  loose  statements  of 
those  who  believe  in  climatic  uniformity.  So  far  as  I  am 
aware,  no  opponent  of  the  hypothesis  of  climatic  changes 
has  ever  even  attempted  to  show  by  careful  statistical 
analysis  that  the  ancient  water  supply  of  such  ruins  was 
no  greater  than  that  of  the  present.  The  most  that  has 
been  done  is  to  suggest  that  there  may  have  been  sources 
of  water  which  are  now  unknown.  Of  course,  this  might 
be  true  in  a  single  instance,  but  it  could  scarcely  be  the 
case  in  many  hundreds  or  thousands  of  ruins. 

5  This  is  due  to  the  fact  that  where  these  forests  occur,  in  Gilead  for 
example,  the  mountains  to  the  west  break  down,  so  that  the  west  winds  with 
water  from  the  Mediterranean  are  able  to  reach  the  inner  range  without 
having  lost  all  their  water.  It  is  one  of  the  misfortunes  of  Syria  that  its 
mountains  generally  rise  so  close  to  the  sea  that  they  shut  off  rainfall  from 
the  interior  and  cause  the  rain  to  fall  on  slopes  too  steep  for  easy  cultivation. 


THE  CLIMATE  OF  HISTORY  71 

Although  the  arguments  in  favor  of  a  change  of  cli- 
mate during  the  last  two  thousand  years  seem  too  strong 
to  be  ignored,  their  very  strength  seems  to  have  been  a 
source  of  error.  A  large  number  of  people  have  jumped 
to  the  conclusion  that  the  change  which  appears  to  have 
occurred  in  certain  regions  occurred  everywhere,  and 
that  it  consisted  of  a  gradual  desiccation. 

Many  observers,  quite  as  careful  as  those  who  believe 
in  progressive  desiccation,  point  to  evidences  of  aridity 
in  past  times  in  the  very  regions  where  the  others  find 
proof  of  moisture.  Lakes  such  as  the  Caspian  Sea  fell  to 
such  a  low  level  that  parts  of  their  present  floors  were 
exposed  and  were  used  as  sites  for  buildings  whose  ruins 
are  still  extant.  Elsewhere,  for  instance  in  the  Tian-Shan 
Mountains,  irrigation  ditches  are  found  in  places  where 
irrigation  never  seems  to  be  necessary  at  present.  In 
Syria  and  North  Africa  during  the  early  centuries  of  the 
Christian  era  the  Romans  showed  unparalleled  activity 
in  building  great  aqueducts  and  in  watering  land  which 
then  apparently  needed  water  almost  as  much  as  it  does 
today.  Evidence  of  this  sort  is  abundant  and  is  as  con- 
vincing as  is  the  evidence  of  moister  conditions  in  the 
past.  It  is  admirably  set  forth,  for  example,  in  the  com- 
prehensive and  ably  written  monograph  of  Leiter  on  the 
climate  of  North  Africa.6  The  evidence  cited  there  and 
elsewhere  has  led  many  authors  strongly  to  advocate  the 
hypothesis  of  climatic  uniformity.  They  have  done  ex- 
actly as  have  the  advocates  of  progressive  change,  and 
have  extended  their  conclusions  over  the  whole  world  and 
over  the  whole  of  historic  times. 

The  hypotheses  of  climatic  uniformity  and  of  progres- 

8  H.  Leiter :  Die  Frage  der  Klimaandernng  waherend  geschichtlicher  Zeit 
in  Nordafrika.  Abhandl.  K.  K.  Geographischen  Gesellschaft,  Wien,  1909, 
p.  143. 


72  CLIMATIC  CHANGES 

sive  change  both  seem  to  be  based  on  reliable  evidence. 
They  may  seem  to  be  diametrically  opposed  to  one 
another,  but  this  is  only  when  there  is  a  failure  to  group 
the  various  lines  of  evidence  according  to  their  dates,  and 
according  to  the  types  of  climate  in  which  they  happen 
to  be  located.  When  the  facts  are  properly  grouped  in 
both  time  and  space,  it  appears  that  evidence  of  moist 
conditions  in  the  historic  Mediterranean  lands  is  found 
during  certain  periods ;  for  instance,  four  or  five  hundred 
years  before  Christ,  at  the  time  of  Christ,  and  1000  A.  D. 
The  other  kind  of  evidence,  on  the  contrary,  culminates 
at  other  epochs,  such  as  about  1200  B.  C.  and  in  the 
seventh  and  thirteenth  centuries  after  Christ.  It  is  also 
found  during  the  interval  from  the  culmination  of  a  moist 
epoch  to  the  culmination  of  a  dry  one,  for  at  such  times 
the  climate  was  growing  drier  and  the  people  were  under 
stress.  This  was  seemingly  the  case  during  the  period 
from  the  second  to  the  fourth  centuries  of  our  era.  North 
Africa  and  Syria  must  then  have  been  distinctly  better 
watered  than  at  present,  as  appears  from  Butler's  vivid 
description ;  but  they  were  gradually  becoming  drier,  and 
the  natural  effect  on  a  vigorous,  competent  people  like  the 
Romans  was  to  cause  them  to  construct  numerous  engi- 
neering works  to  provide  the  necessary  water. 

The  considerations  which  have  just  been  set  forth  have 
led  to  a  third  hypothesis,  that  of  pulsatory  climatic 
changes.  According  to  this,  the  earth's  climate  is  not 
stable,  nor  does  it  change  uniformly  in  one  direction.  It 
appears  to  fluctuate  back  and  forth  not  only  in  the  little 
waves  which  we  see  from  year  to  year  or  decade  to 
decade,  but  in  much  larger  waves,  which  take  hundreds  of 
years  or  even  a  thousand.  These  in  turn  seem  to  merge 
into  and  be  imposed  on  the  greater  waves  which  form 
glacial  stages,  glacial  epochs,  and  glacial  periods.  At  the 


THE  CLIMATE  OF  HISTORY  73 

present  time  there  seems  to  be  no  way  of  determining 
whether  the  general  tendency  is  toward  aridity  or  toward 
glaciation.  The  seventh  century  of  our  era  was  appar- 
ently the  driest  time  during  the  historic  period — distinctly 
drier  than  the  present — but  the  thirteenth  century  was 
almost  equally  dry,  and  the  twelfth  or  thirteenth  before 
Christ  may  have  been  very  dry. 

The  best  test  of  an  hypothesis  is  actual  measurements. 
In  the  case  of  the  pulsatory  hypothesis  we  are  fortu- 
nately able  to  apply  this  test  by  means  of  trees.  The 
growth  of  vegetation  depends  on  many  factors — soil,  ex- 
posure, wind,  sun,  temperature,  rain,  and  so  forth.  In  a 
dry  region  the  most  critical  factor  in  determining  how  a 
tree 's  growth  shall  vary  from  year  to  year  is  the  supply 
of  moisture  during  the  few  months  of  most  rapid 
growth.7  The  work  of  Douglass8  and  others  has  shown 
that  in  Arizona  and  California  the  thickness  of  the 
annual  rings  affords  a  reliable  indication  of  the  amount 
of  moisture  available  during  the  period  of  growth.  This 
is  especially  true  when  the  growth  of  several  years  is 
taken  as  the  unit  and  is  compared  with  the  growth  of 
a  similar  number  of  years  before  or  after.  Where  a  long 
series  of  years  is  used,  it  is  necessary  to  make  corrections 
to  eliminate  the  effects  of  age,  but  this  can  be  done  by 
mathematical  methods  of  considerable  accuracy.  It  is 
difficult  to  determine  whether  the  climate  at  the  beginning 

7  A  most  careful  and  convincing  study  of  this  problem  is  embodied  in 
an  article  by  J.  W.  Smith:  The  Effects  of  Weather  upon  the  Yield  of  Corn; 
Monthly  Weather  Eeview,  Vol.  42,  1914,  pp.  78-92.  On  the  basis  of  the 
yield  of  corn  in  Ohio  for  60  years  and  in  other  states  for  shorter  periods, 
he  shows  that  the  rainfall  of  July  has  almost  as  much  influence  on  the  crop 
as  has  the  rainfall  of  all  other  months  combined.  See  his  Agricultural 
Meteorology,  New  York,  1920. 

s  See  chapter  by  A.  E.  Douglass  in  The  Climatic  Factor ;  and  his  book  on 
Climatic  Cycles  and  Tree-Growth;  Carnegie  Inst.,  1919.  Also  article  by 
M.  N.  Stewart:  The  Relation  of  Precipitation  to  Tree  Growth,  in  the 
Monthly  Weather  Eeview.  Vol.  41,  1913. 


74  CLIMATIC  CHANGES 

and  end  of  a  tree's  life  was  the  same,  but  it  is  easily  pos- 
sible to  determine  whether  there  have  been  pulsations 
while  the  tree  was  making  its  growth.  If  a  large  number 
of  trees  from  various  parts  of  a  given  district  all  formed 
thick  rings  at  a  certain  period  and  then  formed  thin  ones 
for  a  hundred  years,  after  which  the  rings  again  become 
thick,  we  seem  to  be  safe  in  concluding  that  the  trees  have 
lived  through  a  long,  dry  period.  The  full  reasons  for  this 
belief  and  details  as  to  the  methods  of  estimating  climate 
from  tree  growth  are  given  in  The  Climatic  Factor. 

The  results  set  forth  in  that  volume  may  be  summa- 
rized as  follows :  During  the  years  1911  and  1912,  under 
the  auspices  of  the  Carnegie  Institution  of  Washington, 
measurements  were  made  of  the  thickness  of  the  rings  of 
growth  on  the  stumps  of  about  450  sequoia  trees  in  Cali- 
fornia. These  trees  varied  in  age  from  250  to  nearly 
3250  years.  The  great  majority  were  over  1000  years  of 
age,  seventy-nine  were  over  2000  years,  and  three  over 
3000.  Even  where  only  a  few  trees  are  available  the 
record  is  surprisingly  reliable,  except  where  occasional 
accidents  occur.  Where  the  number  approximates  100, 
accidental  variations  are  largely  eliminated  and  we  may 
accept  the  record  with  considerable  confidence.  Accord- 
ingly, we  may  say  that  in  California  we  have  a  fairly 
accurate  record  of  the  climate  for  2000  years  and  an 
approximate  record  for  1000  years  more.  The  final  re- 
sults of  the  measurements  of  the  California  trees  are 
shown  in  Fig.  4,  where  the  climatic  variations  for  3000 
years  in  California  are  indicated  by  the  solid  line.  The 
high  parts  of  the  line  indicate  rainy  conditions,  the  low 
parts,  dry.  An  examination  of  this  curve  shows  that 
during  3000  years  there  have  apparently  been  climatic 
variations  more  important  than  any  which  have  taken 
place  during  the  past  century.  In  order  to  bring  out  the 


.-§-.8 


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76  CLIMATIC  CHANGES 

details  more  clearly,  the  more  reliable  part  of  the  Cali- 
fornia curve,  from  100  B.  C.  to  the  present  time,  has  been 
reproduced  in  Fig.  5.  This  is  identical  with  the  corre- 
sponding part  of  Fig.  4,  except  that  the  vertical  scale  is 
three  times  as  great. 

The  curve  of  tree  growth  in  California  seems  to  be  a 
true  representation  of  the  general  features  of  climatic 
pulsations  in  the  Mediterranean  region.  This  conclusion 
was  originally  based  on  the  resemblance  between  the 
solid  line  of  Fig.  4,  representing  tree  growth,  and  the 
dotted  line  representing  changes  of  climate  in  the  eastern 
Mediterranean  region  as  inferred  from  the  study  of  ruins 
and  of  history  before  any  work  on  this  subject  had  been 
done  in  America.9  The  dotted  line  is  here  reproduced  for 
its  historical  significance  as  a  stage  in  the  study  of  cli- 
matic changes.  If  it  were  to  be  redrawn  today  on  the 
basis  of  the  knowledge  acquired  in  the  last  twelve  years, 
it  would  be  much  more  like  the  tree  curve.  For  example, 
the  period  of  aridity  suggested  by  the  dip  of  the  dotted 
line  about  300  A.  D.  was  based  largely  on  Professor 
Butler's  data  as  to  the  paucity  of  inscriptions  and  ruins 
dating  from  that  period  in  Syria.  In  the  recent  article, 
from  which  a  long  quotation  has  been  given,  he  shows 
that  later  work  proves  that  there  is  no  such  paucity.  On 
the  other  hand,  it  has  accentuated  the  marked  and  sudden 
decay  in  civilization  and  population  which  occurred 
shortly  after  600  A.  D.  He  reached  the  same  conclusion 
to  which  the  present  authors  had  come  on  wholly  different 
grounds,  namely,  that  the  dip  in  the  dotted  line  about  300 
A.  D.  is  not  warranted,  whereas  the  dip  about  630  A.  D. 
is  extremely  important.  In  similar  fashion  the  work  of 

9  The  dotted  line  is  taken  from  Palestine  and  Its  Transformation,  pp. 
327  and  403. 


78  CLIMATIC  CHANGES 

Stein10  in  central  Asia  makes  it  clear  that  the  contrast 
between  the  water  supply  about  200  B.  C.  and  in  the  pre- 
ceding and  following  centuries  was  greater  than  was 
supposed  on  the  basis  of  the  scanty  evidence  available 
when  the  dotted  line  of  Fig.  4  was  drawn  in  1910. 

Since  the  curve  of  the  California  trees  is  the  only  con- 
tinuous and  detailed  record  yet  available  for  the  climate 
of  the  last  three  thousand  years,  it  deserves  most  careful 
study.  It  is  especially  necessary  to  determine  the  degree 
of  accuracy  with  which  the  growth  of  the  trees  repre- 
sents (1)  the  local  rainfall  and  (2)  the  rainfall  of  remote 
regions  such  as  Palestine.  Perhaps  the  best  way  to  deter- 
mine these  matters  is  the  standard  mathematical  method 
of  correlation  coefficients.  If  two  phenomena  vary  in 
perfect  unison,  as  in  the  case  of  the  turning  of  the  wheels 
and  the  progress  of  an  automobile  when  the  brakes  are 
not  applied,  the  correlation  coefficient  is  1.00,  being  posi- 
tive when  the  automobile  goes  forward  and  negative 
when  it  goes  backward.  If  there  is  no  relation  between 
two  phenomena,  as  in  the  case  of  the  number  of  miles  run 
by  a  given  automobile  each  year  and  the  number  of 
chickens  hatched  in  the  same  period,  the  coefficient  is 
zero.  A  partial  relationship  where  other  factors  enter 
into  the  matter  is  represented  by  a  coefficient  between 
zero  and  one,  as  in  the  case  of  the  movement  of  the  auto- 
mobile and  the  consumption  of  gasoline.  In  this  case  the 
relation  is  very  obvious,  but  is  modified  by  other  factors, 
including  the  roughness  and  grade  of  the  road,  the 
amount  of  traffic,  the  number  of  stops,  the  skill  of  the 
driver,  the  condition  and  load  of  the  automobile,  and  the 
state  of  the  weather.  Such  partial  relationships  are  the 
kind  for  which  correlation  coefficients  are  most  useful, 
for  the  size  of  the  coefficients  shows  the  relative  im- 

10  M.  A.  Stein :  Euins  of  Desert  Cathay,  London,  1912. 


THE  CLIMATE  OF  HISTORY  79 

portance  of  the  various  factors.  A  correlation  coefficient 
four  times  the  probable  error,  which  can  always  be  deter-      1 


mined  by  a  formula  well  known  to  mathematicians, 
generally  considered  to  afford  evidence  of  some  kind  of  x 
relation  between  two  phenomena.  When  the  ratio  between 
coefficient  and  error  rises  to  six,  the  relationship  is  re- 
garded as  strong. 

Few  people  would  question  that  there  is  a  connection 
between  tree  growth  and  rainfall,  especially  in  a  climate 
with  a  long  summer  dry  season  like  that  of  California. 
But  the  growth  of  the  trees  also  depends  on  their  posi- 
tion, the  amount  of  shading,  the  temperature,  insect  pests, 
blights,  the  wind  with  its  tendency  to  break  the  branches, 
and  a  number  of  other  factors.  Moreover,  while  rain 
commonly  favors  growth,  great  extremes  are  relatively 
less  helpful  than  more  moderate  amounts.  Again,  the 
roots  of  a  tree  may  tap  such  deep  sources  of  water  that 
neither  drought  nor  excessive  rain  produces  much  effect 
for  several  years.  Hence  in  comparing  the  growth  of  the 
huge  sequoias  with  the  rainfall  we  should  expect  a  corre- 
lation coefficient  high  enough  to  be  convincing,  but  de- 
cidedly below  1.00.  Unfortunately  there  is  no  record  of 
the  rainfall  where  the  sequoias  grow,  the  nearest  long 
record  being  that  of  Sacramento,  nearly  200  miles  to  the 
northwest  and  close  to  sea  level  instead  of  at  an  altitude 
of  about  6000  feet. 

Applying  the  method  of  correlation  coefficients  to  the 
annual  rainfall  of  Sacramento  and  the  growth  of  the 
sequoias  from  1863  to  1910,  we  obtain  the  results  shown 
in  Table  3.  The  trees  of  Section  A  of  the  table  grew  in 
moderately  dry  locations  although  the  soil  was  fairly 
deep,  a  condition  which  seems  to  be  essential  to  sequoias. 
In  this  case,  as  in  all  the  others,  the  rainfall  is  reckoned 
from  July  to  June,  which  practically  means  from  October 


TABLE  3 

CORRELATION  COEFFICIENTS  BETWEEN 

RAINFALL  AND  GROWTH  OF  SEQUOIAS 

IN  CALIFORNIA11 

A.  SACRAMENTO  BAINFALL  AND  GROWTH  OF  18  SEQUOIAS  IN  DRY 
LOCATIONS,  1861-1910 


(r)  (e) 

1  year  of  rainfall    — 0.059     ±0.096  0.6 

2  years  of  rainfall   +0.288     ±0.090  3.2 

3  years  of  rainfall  +0.570     ±0.066  8.7 

4  years  of  rainfall  +0.470     ±0.076  6.2 

B.  SACRAMENTO  EAINFALL  AND  GROWTH  OF  112  SEQUOIAS  MOSTLY  IN 
MOIST  LOCATIONS,  1861-1910 

3  years  of  rainfall  +0.340     ±0.087  3.9 

4  years  of  rainfall  +0.371     ±0.084  4.5 

5  years  of  rainfall  +°-398     ±0.082  4.9 

6  years  of  rainfall  +0.418     ±0.079  5.3 

7  years  of  rainfall  +0.471     ±0.076  6.2 

8  years  of  rainfall  (+0.520)   ±0.071  7.3 

9  years  of  rainfall  +°-575     ±0-065  8.8 

10  years  of  rainfall  +0.577     ±0.065  8.8 

C.  SACRAMENTO  EAINFALL  AND  GROWTH  OF  80  SEQUOIAS  IN  MOIST 

LOCATIONS,  1861-1910 

10  years  of  rainfall  +0.605     ±0.062  9.8 

D.  ANNUAL  SEQUOIA  GROWTH  AND  EAINFALL  OF  PRECEDING  5  YEARS 
AT  STATIONS  ON  SOUTHERN  PACIFIC  EAILROAD 


Sacramento,     1861-1910 

Colfax, 

Summit, 

Truckee, 

Boca, 


70  19.40  200  +0.398 

1871-1909     2400  48.94  200  +0.122 

1871-1909     7000  48.07  200  +0.148 

1871-1909     5800  27.12  200  +0.300 

1871-1909     5500  20.34  200  +0.604 


Winnemucca,    1871-1909     4300       8.65     300     +0.492 


11  In  the  preparation  and  interpretation  of  this  table  the  help  of  Mr. 
G.  B.  Cressey  is  gratefully  acknowledged. 


THE  CLIMATE  OF  HISTORY  81 

to  May,  since  there  is  almost  no  summer  rain.  Thus  the 
tree  growth  in  1861  is  compared  with  the  rainfall  of  the 
preceding  rainy  season,  1860-1861,  or  of  several  preced- 
ing rainy  seasons  as  the  table  indicates. 

In  the  first  line  of  Section  A  a  correlation  coefficient 
of  only  — 0.056,  which  is  scarcely  six-tenths  of  the  prob- 
able error,  means  that  there  is  no  appreciable  relation 
between  the  rainfall  of  a  given  season  and  the  growth 
during  the  following  spring  and  summer.  The  roots  of 
the  sequoias  probably  penetrate  so  deeply  that  the  rain 
and  melted  snow  of  the  spring  months  do  not  sink  down 
rapidly  enough  to  influence  the  trees  before  the  growing 
season  comes  to  an  end.  The  precipitation  of  two  pre- 
ceding seasons,  however,  has  some  effect  on  the  trees,  as 
appears  in  the  second  line  of  Section  A,  where  the  corre- 
lation coefficient  is  +0.288,  or  3.2  times  the  probable 
error.  When  the  rainfall  of  three  seasons  is  taken  into 
account  the  coefficient  rises  to  +0.570,  or  8.7  times  the 
probable  error,  while  with  four  years  of  rainfall  the  coeffi- 
cient begins  to  fall  off.  Thus  the  growth  of  these  eighteen 
sequoias  on  relatively  dry  slopes  appears  to  have  de- 
pended chiefly  on  the  rainfall  of  the  second  and  third 
preceding  rainy  seasons.  The  growth  in  1900,  for 
example,  depended  largely  on  the  rainfall  in  the  rainy 
seasons  of  1897-1898  and  1898-1899. 

Section  B  of  the  table  shows  that  with  112  trees,  grow- 
ing chiefly  in  moist  depressions  where  the  water  supply 
is  at  a  maximum,  the  correlation  between  growth  and 
rainfall,  +0.577  for  ten  years'  rainfall,  is  even  higher 
than  with  the  dry  trees.  The  seepage  of  the  underground 
water  is  so  slow  that  not  until  four  years'  rainfall  is 
taken  into  account  is  the  correlation  coefficient  more  than 
four  times  the  probable  error.  When  only  the  trees  grow- 
ing in  moist  locations  are  employed,  the  coefficient  be- 


82  CLIMATIC  CHANGES 

tween  tree  growth  and  the  rainfall  for  ten  years  rises  to 
the  high  figure  of  +0.605,  or  9.8  times  the  probable  error, 
as  appears  in  Section  C.  These  figures,  as  well  as  many 
others  not  here  published,  make  it  clear  that  the  curve  of 
sequoia  growth  from  1861  to  1910  affords  a  fairly  close 
indication  of  the  rainfall  at  Sacramento,  provided  allow- 
ance be  made  for  a  delay  of  three  to  ten  years  due  to  the 
fact  that  the  moisture  in  the  soil  gradually  seeps  down 
the  mountain-sides  and  only  reaches  the  sequoias  after  a 
considerable  interval. 

If  a  rainfall  record  were  available  for  the  place  where 
the  trees  actually  grow,  the  relationship  would  probably 
be  still  closer. 

The  record  at  Fresno,  for  example,  bears  out  this  con- 
clusion so  far  as  it  goes.  But  as  Fresno  lies  at  a  low  alti- 
tude and  its  rainfall  is  of  essentially  the  Sacramento 
type,  its  short  record  is  of  less  value  than  that  of  Sacra- 
mento. The  only  rainfall  records  among  the  Sierras  at 
high  levels,  where  the  rainfall  and  temperature  are  ap- 
proximately like  those  of  the  sequoia  region,  are  found 
along  the  main  line  of  the  Southern  Pacific  railroad.  This 
runs  from  Oakland  northeastward  seventy  miles  across 
the  open  plain  to  Sacramento,  then  another  seventy  miles, 
as  the  crow  flies,  through  Colfax  and  over  a  high  pass 
in  the  Sierras  at  Summit,  next  twenty  miles  or  so  down 
through  Truckee  to  Boca,  on  the  edge  of  the  inland  basin 
of  Nevada,  and  on  northeastward  another  160  miles  to 
Winnemucca,  where  it  turns  east  toward  Ogden  and  Salt 
Lake  City.  Section  D  of  Table  3  shows  the  correlation 
coefficients  between  the  rainfall  along  the  railroad  and 
the  growth  of  the  sequoias.  At  Sacramento,  which  lies 
fairly  open  to  winds  from  the  Pacific  and  thus  represents 
the  general  climate  of  central  California,  the  coefficient 
is  nearly  five  times  the  probable  error,  thus  indicating  a 


THE  CLIMATE  OF  HISTORY  83 

real  relation  to  sequoia  growth.  Then  among  the  foothills 
of  the  Sierras  at  Colfax,  the  coefficient  drops  till  it  is 
scarcely  larger  than  the  probable  error.  It  rises  rapidly, 
however,  as  one  advances  among  the  mountains,  until  at 
Boca  it  attains  the  high  figure  of  +0.604  or  eight  times 
the  probable  error,  and  continues  high  in  the  dry  area 
farther  east.  In  other  words  the  growth  of  the  sequoias 
is  a  good  indication  of  the  rainfall  where  the  trees  grow 
and  in  the  dry  region  farther  east. 

In  order  to  determine  the  degree  to  which  the  sequoia 
record  represents  the  rainfall  of  other  regions,  let  us 
select  Jerusalem  for  comparison.  The  reasons  for  this 
selection  are  that  Jerusalem  furnishes  the  only  available 
record  that  satisfies  the  following  necessary  conditions : 
(1)  its  record  is  long  enough  to  be  important;  (2)  it  is 
located  fairly  near  the  latitude  of  the  sequoias,  32°N 
versus  37°N;  (3)  it  is  located  in  a  similar  type  of  climate 
with  winter  rains  and  a  long  dry  summer;  (4)  it  lies  well 
above  sea  level  (2500  feet)  and  somewhat  back  from  the 
seacoast,  thus  approximating  although  by  no  means 
duplicating  the  condition  of  the  sequoias;  and  (5)  it  lies 
in  a  region  where  the  evidence  of  climatic  changes  during 
historic  times  is  strongest.  The  ideal  place  for  comparison 
would  be  the  valley  in  which  grow  the  cedars  of  Lebanon. 
Those  trees  resemble  the  sequoias  to  an  extraordinary 
degree,  not  only  in  their  location,  but  in  their  great  age. 
Some  day  it  will  be  most  interesting  to  compare  the 
growth  of  these  two  famous  groups  of  old  trees. 

The  correlation  coefficients  for  the  sequoia  growth 
and  the  rainfall  at  Jerusalem  are  given  in  Section  A, 
Table  4.  They  are  so  high  and  so  consistent  that  they 
scarcely  leave  room  for  doubt  that  where  a  hundred  or 
more  sequoias  are  employed,  as  in  Fig.  5,  their  curve  of 
growth  affords  a  good  indication  of  the  fluctuations  of 


TABLE  4 

CORRELATION  COEFFICIENTS  BETWEEN 
RAINFALL  RECORDS  IN  CALI- 
FORNIA AND  JERUSALEM 

A.  JERUSALEM  RAINFALL  FOR  3  YEARS  AND  VARIOUS  GROUPS  OP 

SEQUOIAS12 


11  trees  measured  by  Douglass   

80  trees,   moist  locations,   Groups  IA, 

IIA,  IIIA,  VA   

101  trees,  69  in  moist  locations,  32  in 

dry,  I,  II,  III 

112  trees,  80  in  moist  locations,  32  in 


(r) 
+0.453 

+0.500 


+0.616     ±0.061 


dry,  I,  II,  III,  V   +0.675     ±0.053 


B.  RAINFALL  AT  JERUSALEM  AND  AT  STATIONS  IN  CALIFORNIA  AND 
NEVADA 


-3  years , 


-5  years 


Sacramento, 

Coif  ax, 

Summit, 

Truckee, 

*Boca, 

Winnemucca, 


70 

2400 
7000 
5800 
5500 
4300 


(r) 


© 

1861-1910  +0.386  4.7 

1871-1909  +°-311  3-1 

1871-1909  +0.099  0.9 

1871-1909  +°-229  2-2 


3871-1909     +0.482     6.4 
1871-1909     +0.235     2.2 
San  Bernardino,   1050     1871-1909     +°-275     2-7 


(r) 

+0.352  4.2 

+  0.308  3.0 

+0.248  2.3 

+0.337  3.3 

+0.617  8.6 

+0.260  2.4 

+0.177  1.8 


C.  RAINFALL  FOR 


YEARS  AT  CALIFORNIA  AND  NEVADA  STATIONS, 
1871-1909 


Sacramento  and  San  Bernardino 
San  Bernardino  and  Winnemueca 


(r)  © 

+0.663     10.7 
+0.291       2.8 


12  For  the  tree  data  used  in  these  comparisons,  see  The  Climatic  Factor, 
p.  328,  and  A.  E.  Douglass:  Climatic  Cycles  and  Tree  Growth,  p.  123. 
*  One  year  interpolated. 


THE  CLIMATE  OF  HISTORY  85 

climate  in  western  Asia.  The  high  coefficient  for  the 
eleven  trees  measured  by  Douglass  suggests  that  where 
the  number  of  trees  falls  as  low  as  ten,  as  in  the  part  of 
Fig.  4  from  710  to  840  B.  C.,  the  relation  between  tree 
growth  and  rainfall  is  still  close  even  when  only  one 
year's  growth  is  considered.  Where  the  unit  is  ten  years 
of  growth,  as  in  Figs.  4  and  5,  the  accuracy  of  the  tree 
curve  as  a  measure  of  rainfall  is  much  greater  than  when 
a  single  year  is  used  as  in  Table  4.  When  the  unit  is 
raised  to  thirty  years,  as  in  the  smoothed  part  of  Fig.  4 
previous  to  240  B.  C.,  even  four  trees,  as  from  960  to 
1070,  probably  give  a  fair  approximation  to  the  general 
changes  in  rainfall,  while  a  single  tree  prior  to  1110  B.  C. 
gives  a  rough  indication. 

Table  4  shows  a  peculiar  feature  in  the  fact  that  the 
correlations  of  Section  A  between  tree  growth  and  the 
rainfall  of  Jerusalem  are  decidedly  higher  than  those 
between  the  rainfall  in  the  two  regions.  Only  at  Sacra- 
mento and  Boca  are  the  rainfall  coefficients  high  enough 
to  be  conclusive.  This,  however,  is  not  surprising,  for 
even  between  Sacramento  and  San  Bernardino,  only  400 
miles  apart,  the  correlation  coefficient  for  the  rainfall 
by  three-year  periods  is  only  10.7  times  the  probable 
error,  as  appears  in  Section  C  of  Table  4,  while  between 
San  Bernardino  and  Winnemucca  500  miles  away,  the 
corresponding  figure  drops  to  2.8.  It  must  be  remem- 
bered that  in  some  respects  the  growth  of  the  sequoias  is 
a  much  better  record  of  rainfall  than  are  the  records  kept 
by  man.  The  human  record  is  based  on  the  amount  of 
water  caught  by  a  little  gauge  a  few  inches  in  diameter. 
Every  gust  of  wind  detracts  from  the  accuracy  of  the 
record ;  a  mile  away  the  rainfall  may  be  double  what  it 
is  at  the  gauge.  Each  sequoia,  on  the  other  hand,  draws 
its  moisture  from  an  area  thousands  of  times  as  large  as 


86  CLIMATIC  CHANGES 

a  rain  gauge.  Moreover,  the  trees  on  which  Figs.  4  and  5 
are  based  were  scattered  over  an  area  fifty  miles  long 
and  several  hundred  square  miles  in  extent.  Hence  they 
represent  the  summation  of  the  rainfall  over  an  area 
millions  of  times  as  large  as  that  of  a  rain  gauge.  This 
fact  and  the  large  correlation  coefficients  between  sequoia 
growth  and  Jerusalem  rainfall  should  be  considered  in 
connection  with  the  fact  that  all  the  coefficients  between 
the  rainfall  of  California  and  Nevada  and  that  of  Jeru- 
salem are  positive.  If  full  records  of  the  complete  rainfall 
of  California  and  Nevada  on  the  one  hand  and  of  the 
eastern  Mediterranean  region  on  the  other  were  available 
for  a  long  period,  they  would  probably  agree  closely. 

Just  how  widely  the  sequoias  can  be  used  as  a  measure 
of  the  climate  of  the  past  is  not  yet  certain.  In  some 
regions,  as  will  shortly  be  explained,  the  climatic  changes 
seem  to  have  been  of  an  opposite  character  from  those 
of  California.  In  others  the  Californian  or  eastern  Medi- 
terranean type  of  change  seems  sometimes  to  prevail  but 
is  not  always  evident.  For  example,  at  Malta  the  rainfall 
today  shows  a  distinct  relation  to  that  of  Jerusalem  and 
to  the  growth  of  the  sequoias.  But  the  correlation  coeffi- 
cient between  the  rainfall  of  eight-year  periods  at  Naples, 
a  little  farther  north,  and  the  growth  of  the  sequoias  at 
the  end  of  the  periods  is  — 0.132,  or  only  1.4  times  the 
probable  error  and  much  too  small  to  be  significant.  This 
is  in  harmony  with  the  fact  that  although  Naples  has 
summer  droughts,  they  are  not  so  pronounced  as  in  Cali- 
fornia and  Palestine,  and  the  prevalence  of  storms  is 
much  greater.  Jerusalem  receives  only  8  per  cent  of  its 
rain  in  the  seven  months  from  April  to  October,  and 
Sacramento  13,  while  Malta  receives  31  per  cent  and 
Naples  43.  Nevertheless,  there  is  some  evidence  that  in 
the  past  the  climatic  fluctuations  of  southern  Italy  fol- 


THE  CLIMATE  OF  HISTORY  87 

lowed  nearly  the  same  course  as  those  of  California  and 
Palestine.  This  apparent  discrepancy  seems  to  be  ex- 
plained by  our  previous  conclusion  that  changes  of  cli- 
mate are  due  largely  to  a  shifting  of  storm  tracks.  When 
sunspots  are  numerous  the  storms  which  now  prevail  in 
northern  Italy  seem  to  be  shifted  southward  and  traverse 
the  Mediterranean  to  Palestine  just  as  similar  storms 
are  shifted  southward  in  the  United  States.  This  perhaps 
accounts  for  the  agreement  between  the  sequoia  curve 
and  the  agricultural  and  social  history  of  Rome  from 
about  400  B.  C.  to  100  A.  D.,  as  explained  in  World  Power 
and  Evolution.  For  our  present  purposes,  however,  the 
main  point  is  that  since  rainfall  records  have  been  kept 
the  fluctuations  of  climate  indicated  by  the  growth  of  the 
sequoias  have  agreed  closely  with  fluctuations  in  the 
rainfall  of  the  eastern  Mediterranean  region.  Presumably 
the  same  was  true  in  the  past.  In  that  case,  the  sequoia 
curve  not  only  is  a  good  indication  of  climatic  changes  or 
pulsations  in  regions  of  similar  climate,  but  may  serve 
as  a  guide  to  coincident  but  different  changes  in  regions 
of  other  types. 

An  enormous  body  of  other  evidence  points  to  the  same 
conclusion.  It  indicates  that  while  the  average  climate 
of  the  present  is  drier  than  that  of  the  past  in  regions 
having  the  Mediterranean  type  of  winter  rains  and 
summer  droughts,  there  have  been  pronounced  pulsations 
during  historic  times  so  that  at  certain  times  there  has 
actually  been  greater  aridity  than  at  present.  This  con- 
clusion is  so  important  that  it  seems  advisable  to  examine 
the  only  important  arguments  that  have  been  raised 
against  it,  especially  against  the  idea  that  the  general 
rainfall  of  the  eastern  Mediterranean  was  greater  in  the 
historic  past  than  at  present.  The  first  objection  is  the 
unquestionable  fact  that  droughts  and  famines  have 


88  CLIMATIC  CHANGES 

occurred  at  periods  which  seem  on  other  evidence  to  have 
been  moister  than  the  present.  This  argument  has  been 
much  used,  but  it  seems  to  have  little  force.  If  the  rain- 
fall of  a  given  region  averages  thirty  inches  and  varies 
from  fifteen  to  forty-five,  a  famine  will  ensue  if  the  rain- 
fall drops  for  a  few  years  to  the  lower  limit  and  does  not 
rise  much  above  twenty  for  a  few  years.  If  the  climate  of 
the  place  changes  during  the  course  of  centuries,  so  that 
the  rainfall  averages  only  twenty  inches,  and  ranges 
from  seven  to  thirty-five,  famine  will  again  ensue  if  the 
rainfall  remains  near  ten  inches  for  a  few  years.  The 
ravages  of  the  first  famine  might  be  as  bad  as  those  of 
the  second.  They  might  even  be  worse,  because  when  the 
rainfall  is  larger  the  population  is  likely  to  be  greater 
and  the  distress  due  to  scarcity  of  food  would  affect  a 
larger  number  of  people.  Hence  historic  records  of 
famines  and  droughts  do  not  indicate  that  the  climate 
was  either  drier  or  moister  than  at  present.  They  merely 
show  that  at  the  time  in  question  the  climate  was  drier 
than  the  normal  for  that  particular  period. 

The  second  objection  is  that  deserts  existed  in  the  past 
much  as  at  present.  This  is  not  a  real  objection,  however, 
for,  as  we  shall  see  more  fully,  some  parts  of  the  world 
suffer  one  kind  of  change  and  others  quite  the  opposite. 
Moreover,  deserts  have  always  existed,  and  when  we  talk 
of  a  change  in  their  climate  we  merely  mean  that  their 
boundaries  have  shifted.  A  concrete  example  of  the  mis- 
taken use  of  ancient  dryness  as  proof  of  climatic  uni- 
formity is  illustrated  by  the  march  of  Alexander  from 
India  to  Mesopotamia.  Hedin  gives  an  excellent  presen- 
tation of  the  case  in  the  second  volume  of  his  Overland 
to  India.  He  shows  conclusively  that  Alexander's  army 
suffered  terribly  from  lack  of  water  and  provisions.  This 
certainly  proves  that  the  climate  was  dry,  but  it  by  no 


THE  CLIMATE  OF  HISTORY  89 

means  indicates  that  there  has  been  no  change  from  the 
past  to  the  present.  We  do  not  know  whether  Alexander's 
march  took  place  during  an  especially  dry  or  an  espe- 
cially wet  year.  In  a  desert  region  like  Makran,  in 
southern  Persia  and  Beluchistan,  where  the  chief  diffi- 
culties occurred,  the  rainfall  varies  greatly  from  year  to 
year.  We  have  no  records  from  Makran,  but  the  condi- 
tions there  are  closely  similar  to  those  of  southern 
Arizona  and  New  Mexico.  In  1885  and  1905  the  rainfall 
for  five  stations  in  that  region  was  as  follows : 


Mean  rainfall  dur- 

1885 

1905 

ing  period  since 
observations 

*/Yuma,  Arizona, 

2.72 

11.41 

began 
3.13 

Phoenix,  Arizona, 

3.77 

19.73 

7.27 

i/Tueson,  Arizona, 

5.26 

24.17 

11.66 

Clx>rdsburg,  New  Mexico, 

3.99 

19.50 

8.62 

JEL  Paso,  Texas  (on  New 

Mexico  border), 

7.31 

17.80 

9.06 

Average, 

4.61 

18.52 

7.95 

These  stations  are  distributed  over  an  area  nearly  500 
miles  east  and  west.  Manifestly  a  traveler  who  spent  the 
year  1885  in  that  region  would  have  had  much  more  diffi- 
culty in  finding  water  and  forage  than  one  who  traveled 
in  the  same  places  in  1905.  During  1885  the  rainfall  was 
42  per  cent  less  than  the  average,  and  during  1905  it  was 
134  per  cent  more  than  the  average.  Let  us  suppose,  for 
the  sake  of  argument,  that  the  average  rainfall  of  south- 
eastern Persia  is  six  inches  today  and  was  ten  inches  in 
the  days  of  Alexander.  If  the  rainfall  from  year  to  year 
varied  as  much  in  the  past  in  Persia  as  it  does  now  in 
New  Mexico  and  Arizona,  the  rainfall  during  an  ancient 


90  CLIMATIC  CHANGES 

dry  year,  corresponding  in  character  to  1885,  would  have 
been  about  5.75  inches.  On  the  other  hand,  if  we  suppose 
that  the  rainfall  then  averaged  less  than  at  present, — let 
us  say  four  inches, — a  wet  year  corresponding  to  1905  in 
the  American  deserts  might  have  had  a  rainfall  of  about 
ten  inches.  This  being  the  case,  it  is  clear  that  our  esti- 
mate of  what  Alexander's  march  shows  as  to  climate 
must  depend  largely  on  whether  325  B.  C.  was  a  wet  year 
or  a  dry  year.  Inasmuch  as  we  know  nothing  about  this, 
we  must  fall  back  on  the  fact  that  a  large  army  accom- 
plished a  journey  in  a  place  where  today  even  a  small 
caravan  usually  finds  great  difficulty  in  procuring  forage 
and  water.  Moreover,  elephants  were  taken  180  miles 
across  what  is  now  an  almost  waterless  desert,  and  yet 
the  old  historians  make  no  comment  on  such  a  feat  which 
today  would  be  practically  impossible.  These  things  seem 
more  in  harmony  with  a  change  of  climate  than  with 
uniformity.  Nevertheless,  it  is  not  safe  to  place  much 
reliance  on  them  except  when  they  are  taken  in  con- 
junction with  other  evidence,  such  as  the  numerous  ruins, 
which  show  that  Makran  was  once  far  more  densely 
populated  than  now  seems  possible.  Taken  by  itself,  such 
incidents  as  Alexander's  march  cannot  safely  be  used 
either  as  an  argument  for  or  against  changes  of  climate. 
The  third  and  strongest  objection  to  any  hypothesis 
of  climatic  changes  during  historic  times  is  based  on 
vegetation.  The  whole  question  is  admirably  set  forth  by 
J.  W.  Gregory,13  who  gives  not  only  his  own  results,  but 
those  of  the  ablest  scholars  who  have  preceded  him.  His 
conclusions  are  important  because  they  represent  one  of 
the  few  cases  where  a  definite  statistical  attempt  has  been 
made  to  prove  the  exact  condition  of  the  climate  of  the 

is  J.  W.  Gregory:  Is  the  Earth  Drying  Up?  Geog.  Jour.,  Vol.  43,  1914, 
pp.  148-172  and  293-318. 


THE  CLIMATE  OF  HISTORY  91 

past.  After  stating  various  less  important  reasons  for 
believing  that  the  climate  of  Palestine  has  not  changed, 
he  discusses  vegetation.  The  following  quotation  indi- 
cates his  line  of  thought.  A  sentence  near  the  beginning 
is  italicized  in  order  to  call  attention  to  the  importance 
which  Gregory  and  others  lay  on  this  particular  kind  of 
evidence : 

Some  more  certain  ^test  is  necessary  than  the  general  con- 
clusions which  can  be  based  upon  the  historical  and  geographical 
evidence  of  the  Bible.  In  the  absence  of  rain  gauge  and  thermo- 
metric  records,  the  most  precise  test  of  climate  is  given  by  the 
vegetation;  and  fortunately  the  palm  affords  a  very  delicate  test 
of  the  past  climate  of  Palestine  and  the  eastern  Mediterranean. 
.  .  .  The  date  palm  has  three  limits  of  growth  which  are  deter- 
mined by  temperature;  thus  it  does  not  reach  full  maturity  or 
produce  ripe  fruit  of  good  quality  below  the  mean  annual  tem- 
perature of  69°F.  The  isothermal  of  69°  crosses  southern  Algeria 
near  Biskra;  it  touches  the  northern  coasts  of  Cyrenaica  near 
Derna  and  passes  Egypt  near  the  mouth  of  the  Nile,  and  then 
bends  northward  along  the  coast  lands  of  Palestine. 

To  the  north  of  this  line  the  date  palm  grows  and  produces 
fruit,  which  only  ripens  occasionally,  and  its  quality  deteriorates 
as  the  temperature  falls  below  69°.  Between  the  isotherms  of 
68°  and  64°,  limits  which  include  northern  Algeria,  most  of 
Sicily,  Malta,  the  southern  parts  of  Greece  and  northern  Syria, 
the  dates  produced  are  so  unripe  that  they  are  not  edible.  In  the 
next  cooler  zone,  north  of  the  isotherm  of  62°,  which  enters 
Europe  in  southwestern  Portugal,  passes  through  Sardinia, 
enters  Italy  near  Naples,  crosses  northern  Greece  and  Asia 
Minor  to  the  east  of  Smyrna,  the  date  palm  is  grown  only  for 
its  foliage,  since  it  does  not  fruit. 

Hence  at  Benghazi,  on  the  north  African  coast,  the  date  palm 
is  fertile,  but  produces  fruit  of  poor  quality.  In  Sicily  and  at 
Algiers  the  fruit  ripens  occasionally  and  at  Rome  and  Nice  the 
palm  is  grown  only  as  an  ornamental  tree. 


92  CLIMATIC  CHANGES 

The  date  palm  therefore  affords  a  test  of  variations  in  mean 
annual  temperature  of  three  grades  between  62°  and  69°. 

This  test  shows  that  the  mean  annual  temperature  of  Palestine 
has  not  altered  since  Old  Testament  times.  The  palm  tree  now 
grows  dates  on  the  coast  of  Palestine  and  in  the  deep  depression 
around  the  Dead  Sea,  but  it  does  not  produce  fruit  on  the  high- 
lands of  Judea.  Its  distribution  in  ancient  times,  as  far  as  we 
can  judge  from  the  Bible,  was  exactly  the  same.  It  grew  at 
' '  Jericho,  the  city  of  palm  trees ' '  (Deut.  xxxiv :  3  and  2  Chron. 
xxviii:  15),  and  at  Engedi,  on  the  western  shore  of  the  Dead 
Sea  (2  Chron.  xx:2;  Sirach  xxiv:14);  and  though  the  palm 
does  not  still  live  at  Jericho — the  last  apparently  died  in  1838 — 
its  disappearance  must  be  due  to  neglect,  for  the  only  climatic 
change  that  would  explain  it  would  be  an  increase  in  cold  or 
moisture.  In  olden  times  the  date  palm  certainly  grew  on  the 
highlands  of  Palestine;  but  apparently  it  never  produced  fruit 
there,  for  the  Bible  references  to  the  palm  are  to  its  beauty  and 
erect  growth:  "The  righteous  shall  nourish  like  the  palm"  (Ps. 
xcii:  12) ;  "They  are  upright  as  the  palm  tree"  (Jer.  x:  5) ; 
"Thy  stature  is  like  to  a  palm  tree"  (Cant,  vii:  7).  It  is  used  as 
a  symbol  of  victory  (Rev.  vii:  9),  but  never  praised  as  a  source 
of  food. 

Dates  are  not  once  referred  to  in  the  text  of  the  Bible,  but 
according  to  the  marginal  notes  the  word  translated  "honey"  in 
2  Chron.  xxxi :  5  may  mean  dates.  .  .  . 

It  appears,  therefore,  that  the  date  palm  had  essentially  the 
same  distribution  in  Palestine  in  Old  Testament  times  as  it  has 
now;  and  hence  we  may  infer  that  the  mean  temperature  was 
then  the  same  as  now.  If  the  climate  had  been  moister  and  cooler, 
the  date  could  not  have  flourished  at  Jericho.  If  it  had  been 
warmer,  the  palms  would  have  grown  freely  at  higher  levels  and 
Jericho  would  not  have  held  its  distinction  as  the  city  of  palm 
trees.14 

In  the  main  Gregory's  conclusions  seem  to  be  well 
grounded,  although  even  according  to  his  data  a  change 

«  Geog.  Jour.,  Vol.  43,  pp.  159-161. 


THE  CLIMATE  OF  HISTORY  93 

of  2°  or  3°  in  mean  temperature  would  be  perfectly 
feasible.  It  will  be  noticed,  however,  that  they  apply  to 
temperature  and  not  to  rainfall.  They  merely  prove  that 
two  thousand  years  ago  the  mean  temperature  of  Pales- 
tine and  the  neighboring  regions  was  not  appreciably  dif- 
ferent from  what  it  is  today.  This,  however,  is  in  no  sense 
out  of  harmony  with  the  hypothesis  of  climatic  pulsa- 
tions. Students  of  glaciation  believe  that  during  the  last 
glacial  epoch  the  mean  temperature  of  the  earth  as  a 
whole  was  only  5°  or  6°C.  lower  than  at  present.  If  the 
difference  between  the  climate  of  today  and  of  the  time  of 
Christ  is  a  tenth  as  great  as  the  difference  between  the 
climate  of  today  and  that  which  prevailed  at  the  culmina- 
tion of  the  last  glacial  epoch,  the  change  in  two  thousand 
years  has  been  of  large  dimensions.  Yet  this  would  re- 
quire a  rise  of  only  half  a  degree  Centigrade  in  the  mean 
temperature  of  Palestine.  Manifestly,  so  slight  a  change 
would  scarcely  be  detectable  in  the  vegetation. 

The  slightness  of  changes  in  mean  temperature  as  com- 
pared with  changes  in  rainfall  may  be  judged  from  a 
comparison  of  wet  and  dry  years  in  various  regions.  For 
example,  at  Berlin  between  1866  and  1905  the  ten  most 
rainy  years  had  an  average  precipitation  of  670  mm.  and 
a  mean  temperature  of  9.15 °C.  On  the  other  hand,  the  ten 
years  of  least  rainfall  had  an  average  of  483  mm.  and  a 
mean  temperature  of  9.35°.  In  other  words,  a  difference 
of  137  mm.,  or  39  per  cent,  in  rainfall  was  accompanied 
by  a  difference  of  only  0.2°C.  in  temperature.  Such  con- 
trasts between  the  variability  of  mean  rainfall  and  mean 
temperature  are  observable  not  only  when  individual 
years  are  selected,  but  when  much  longer  periods  are 
taken.  For  instance,  in  the  western  Gulf  region  of  the 
United  States  the  two  inland  stations  of  Vicksburg,  Mis- 
sissippi, and  Shreveport,  Louisiana,  and  the  two  mari- 


94  CLIMATIC  CHANGES 

time  stations  of  New  Orleans,  Louisiana,  and  Galveston, 
Texas,  lie  at  the  margins  of  an  area  about  400  miles  long. 
During  the  ten  years  from  1875  to  1884  their  rainfall 
averaged  59.4  inches,15  while  during  the  ten  years  from 
1890  to  1899  it  averaged  only  42.4  inches.  Even  in  a 
region  so  well  watered  as  the  Gulf  States,  such  a  change 
— 40  per  cent  more  in  the  first  decade  than  in  the  second 
— is  important,  and  in  drier  regions  it  would  have  a  great 
effect  on  habitability.  Yet  in  spite  of  the  magnitude  of 
the  change  the  mean  temperature  was  not  appreciably 
different,  the  average  for  the  four  stations  being  67.36°F. 
during  the  more  rainy  decade  and  66.94° F.  during  the 
less  rainy  decade — a  difference  of  only  0.42°F.  It  is  worth 
noticing  that  in  this  case  the  wetter  period  was  also  the 
warmer,  whereas  in  Berlin  it  was  the  cooler.  This  is 
probably  because  a  large  part  of  the  moisture  of  the  Gulf 
States  is  brought  by  winds  having  a  southerly  com- 
ponent. Similar  relationships  are  apparent  in  other 
places.  We  select  Jerusalem  because  we  have  been  dis- 
cussing Palestine.  At  the  time  of  writing,  the  data  avail- 
able in  the  Quarterly  Journal  of  the  Palestine  Explora- 
tion Fund  cover  the  years  from  1882-1899  and  1903-1909. 
Among  these  twenty-five  years  the  thirteen  which  had 
most  rain  had  an  average  of  34.1  inches  and  a  tempera- 
ture of  62.04°F.  The  twelve  with  least  rain  had  24.4  inches 
and  a  temperature  of  62.44°.  A  difference  of  40  per  cent 
in  rainfall  was  accompanied  by  a  difference  of  only 
0.4° F.  in  temperature. 

The  facts  set  forth  in  the  preceding  paragraphs  seem 
to  show  that  extensive  changes  in  precipitation  and 
storminess  can  take  place  without  appreciable  changes  of 
mean  temperature.  If  such  changed  conditions  can  per- 

i5  See  A.  J.  Henry:  Secular  Variation  of  Precipitation  in  the  United 
States;  Bull.  Am.  Geog.  Soc.,  Vol.  46,  1914,  pp.  192-201. 


THE  CLIMATE  OF  HISTORY  95 

sist  for  ten  years,  as  in  one  of  our  examples,  there  is  no 
logical  reason  why  they  cannot  persist  for  a  hundred  or 
a  thousand.  The  evidence  of  changes  in  climate  during  the 
historic  period  seems  to  suggest  changes  in  precipitation  . 
much  more  than  in  temperature.  Hence  the  strongest  of 
all  the  arguments  against  historic  changes  of  climate 
seems  to  be  of  relatively  little  weight,  and  the  pulsatory 
hypothesis  seems  to  be  in  accord  with  all  the  known  facts. 
Before  the  true  nature  of  climatic  changes,  whether 
historic  or  geologic,  can  be  rightly  understood,  another 
point  needs  emphasis.  When  the  pulsatory  hypothesis 
was  first  framed,  it  fell  into  the  same  error  as  the  hy- 
potheses of  uniformity  and  of  progressive  change — that 
is,  the  assumption  was  made  that  the  whole  world  is 
either  growing  drier  or  moister  with  each  pulsation.  A 
study  of  the  ruins  of  Yucatan,  in  1912,  and  of  Guatemala, 
in  1913,  as  is  explained  in  The  Climatic  Factor,  has  led  to 
the  conclusion  that  the  climate  of  those  regions  has 
changed  in  the  opposite  way  from  the  changes  which 
appear  to  have  taken  place  in  the  desert  regions  farther 
south.  These  Maya  ruins  in  Central  America  are  in  many 
cases  located  in  regions  of  such  heavy  rainfall,  such  dense 
forests,  and  such  malignant  fevers  that  habitation  is  now 
practically  impossible.  The  land  cannot  be  cultivated 
except  in  especially  favorable  places.  The  people  are 
terribly  weakened  by  disease  and  are  among  the  lowest 
in  Central  America.  Only  a  hundred  miles  from  the  un- 
healthful  forests  we  find  healthful  areas,  such  as  the 
coasts  of  Yucatan  and  the  plateau  of  Guatemala.  Here 
the  vast  majority  of  the  population  is  gathered,  the  large 
towns  are  located,  and  the  only  progressive  people  are 
found.  Nevertheless,  in  the  past  the  region  of  the  forests 
was  the  home  of  by  far  the  most  progressive  people  who 
are  ever  known  to  have  lived  in  America  previous  to  the 


96  CLIMATIC  CHANGES 

days  of  Columbus.  They  alone  brought  to  high  perfection 
the  art  of  sculpture ;  they  were  the  only  American  people 
who  invented  the  art  of  writing.  It  seems  scarcely  credi- 
ble that  such  a  people  would  have  lived  in  the  worst  pos- 
sible habitat  when  far  more  favored  regions  were  close 
at  hand.  Therefore  it  seems  as  if  the  climate  of  eastern 
Guatemala  and  Yucatan  must  have  been  relatively  dry 
at  some  past  time.  The  Maya  chronology  and  traditions 
indicate  that  this  was  probably  at  the  same  time  when 
moister  conditions  apparently  prevailed  in  the  subarid 
or  desert  portions  of  the  United  States  and  Asia.  Fig.  3 
shows  that  today  at  times  of  many  sunspots  there  is 
a  similar  opposition  between  a  tendency  toward  stormi- 
ness  and  rain  in  subtropical  regions  and  toward  aridity 
in  low  latitudes  near  the  heat  equator. 

Thus  our  final  conclusion  is  that  during  historic  times 
there  have  been  pulsatory  changes  of  climate.  These 
changes  have  been  of  the  same  type  in  regions  having 
similar  kinds  of  climate,  but  of  different  and  sometimes 
opposite  types  in  places  having  diverse  climates.  As  to 
the  cause  of  the  pulsations,  they  cannot  have  been  due  to 
the  precession  of  the  equinoxes  nor  apparently  to  any 
allied  astronomical  cause,  for  the  time  intervals  are  too 
short  and  too  irregular.  They  cannot  have  been  due  to 
changes  in  the  percentage  of  carbon  dioxide  in  the  atmos- 
phere, for  not  even  the  strongest  believers  in  the  climatic 
efficacy  of  that  gas  hold  that  its  amount  could  fluctuate  in 
any  such  violent  way  as  would  be  necessary  to  explain 
the  pulsations  shown  in  the  California  curve  of  tree 
growth.  Volcanic  activity  seems  more  probable  as  at  least 
a  partial  cause,  and  it  would  be  worth  while  to  investigate 
the  matter  more  fully.  Nevertheless,  it  can  apparently 
be  only  a  minor  cause.  In  the  first  place,  the  main  effect 
of  a  cloud  of  dust  is  to  alter  the  temperature,  but 


THE  CLIMATE  OF  HISTORY  97 

Gregory's  summary  of  the  palm  and  the  vine  shows  that 
variations  in  temperature  are  apparently  of  very  slight 
importance  during  historic  times.  Again,  ruins  on  the 
bottoms  of  enclosed  salt  lakes,  old  beaches  now  under  the 
water,  and  signs  of  irrigation  ditches  where  none  are  now 
needed  indicate  a  climate  drier  than  the  present.  Vol- 
canic dust,  however,  cannot  account  for  such  a  condi- 
tion, for  at  present  the  air  seems  to  be  practically  free 
from  such  dust  for  long  periods.  Thus  we  now  experience 
the  greatest  extreme  which  the  volcanic  hypothesis  per- 
mits in  one  direction,  but  there  Jiave  been  greater  ex- 
tremes in  the  same  direction.  The  thermal  solar  hypothe- 
sis is  likewise  unable  to  explain  the  observed  phenomena, 
for  neither  it  nor  the  volcanic  hypothesis  offers  any  expla- 
nation of  why  the  climate  varies  in  one  way  in  Medi- 
terranean climates  and  in  an  opposite  way  in  regions 
near  the  heat  equator. 

This  leaves  the  cyclonic  hypothesis.  It  seems  to  fit  the 
facts,  for  variations  in  cyclonic  storms  cause  some 
regions  to  be  moister  and  others  drier  than  usual.  At  the 
same  time  the  variations  in  temperature  are  slight,  and 
are  apparently  different  in  different  regions,  some  places 
growing  warm  when  others  grow  cool.  In  the  next  chap- 
ter we  shall  study  this  matter  more  fully,  for  it  can  best 
be  appreciated  by  examining  the  course  of  events  in  a 
specific  century. 


CHAPTER  VI 

THE  CLIMATIC  STRESS  OF  THE  FOURTEENTH 
CENTURY 

IN  order  to  give  concreteness  to  our  picture  of  the 
climatic  pulsations  of  historic  times  let  us  take  a 
specific  period  and  see  how  its  changes  of  climate 
were  distributed  over  the  globe  and  how  they  are  related 
to  the  little  changes  which  now  take  place  in  the  sunspot 
cycle.  We  will  take  the  fourteenth  century  of  the  Chris- 
tian era,  especially  the  first  half.  This  period  is  chosen 
because  it  is  the  last  and  hence  the  best  known  of  the 
times  when  the  climate  of  the  earth  seems  to  have  taken 
a  considerable  swing  toward  the  conditions  which  now 
prevail  when  the  sun  is  most  active,  and  which,  if  inten- 
sified, would  apparently  lead  to  glaciation.  It  has  already 
been  discussed  in  World  Power  and  Evolution,  but  its 
importance  and  the  fact  that  new  evidence  is  constantly 
coming  to  light  warrant  a  fuller  discussion. 

To  begin  with  Europe ;  according  to  the  careful  account 
of  Pettersson1  the  fourteenth  century  shows 

a  record  of  extreme  climatic  variations.  In  the  cold  winters  the 
rivers  Rhine,  Danube,  Thames,  and  Po  were  frozen  for  weeks 
and  months.  On  these  cold  winters  there  followed  violent  floods, 
so  that  the  rivers  mentioned  inundated  their  valleys.  Such  floods 
are  recorded  in  55  summers  in  the  14th  century.  There  is,  of 

i  O.  Pettersson :  The  connection  between  hydrographical  and  meteorologi- 
cal phenomena;  Quarterly  Journal  of  the  Royal  Meteorological  Society,  Vol. 
38,  pp.  174-175. 


STRESS  OF  FOURTEENTH  CENTURY  99 

course,  nothing  astonishing  in  the  fact  that  the  inundations  of 
the  great  rivers  of  Europe  were  more  devastating  600  to  700 
years  ago  than  in  our  days,  when  the  flow  of  the  rivers  has  been 
regulated  by  canals,  locks,  etc. ;  but  still  the  inundations  in  the 
13th  and  14th  centuries  must  have  surpassed  everything  of  that 
kind  which  has  occurred  since  then.  In  1342  the  waters  of  the 
Rhine  rose  so  high  that  they  inundated  the  city  of  Mayence  and 
the  Cathedral  "  usque  ad  cingulum  hominis."  The  walls  of 
Cologne  were  flooded  so  that  they  could  be  passed  by  boats  in 
July.  This  occurred  also  in  1374  in  the  midst  of  the  month  of 
February,  which  is  of  course  an  unusual  season  for  disasters  of 
the  kind.  Again  in  other  years  the  drought  was  so  intense  that 
the  same  rivers,  the  Danube,  Rhine,  and  others,  nearly  dried  up, 
and  the  Rhine  could  be  forded  at  Cologne.  This  happened  at  least 
twice  in  the  same  century.  There  is  one  exceptional  summer  of 
such  evil  record  that  centuries  afterwards  it  was  spoken  of  as 
"the  old  hot  summer  of  1357." 

Pettersson  goes  on  to  speak  of  two  oceanic  phenomena 
on  which  the  old  chronicles  lay  greater  stress  than  on 
all  others : 

The  first  [is]  the  great  storm-floods  on  the  coast  of  the  North 
Sea  and  the  Baltic,  which  occurred  so  frequently  that  not  less 
than  nineteen  floods  of  a  destructiveness  unparalleled  in  later 
times  are  recorded  from  the  14th  century.  The  coastline  of  the 
North  Sea  was  completely  altered  by  these  floods.  Thus  on 
January  16,  1300,  half  of  the  island  Heligoland  and  many  other 
islands  were  engulfed  by  the  sea.  The  same  fate  overtook  the 
island  of  Borkum,  torn  into  several  islands  by  the  storm-flood  of 
January  16,  which  remoulded  the  Frisian  Islands  into  their 
present  shape,  when  also  Wendingstadt,  on  the  island  of  Sylt, 
and  Thiryu  parishes  were  engulfed.  This  flood  is  known  under 
the  name  of  ' '  the  great  man-drowning. ' '  The  coasts  of  the  Baltic 
also  were  exposed  to  storm-floods  of  unparalleled  violence.  On 
November  1,  1304,  the  island  of  Ruden  was  torn  asunder  from 
Rugen  by  the  force  of  the  waves.  Time  does  not  allow  me  to 
dwell  upon  individual  disasters  of  this  kind,  but  it  will  be  well 


100  CLIMATIC  CHANGES 

to  note  that  of  the  nineteen  great  floods  on  record  eighteen 
occurred  in  the  cold  season  between  the  autumnal  and  vernal 
equinoxes. 

The  second  remarkable  phenomenon  mentioned  by  the  chron- 
icles is  the  freezing  of  the  entire  Baltic,  which  occurred  many 
times  during  the  cold  winters  of  these  centuries.  On  such  occa- 
sions it  was  possible  to  travel  with  carriages  over  the  ice  from 
Sweden  to  Bornholm  and  from  Denmark  to  the  German  coast 
(Lubeck),  and  in  some  cases  even  from  Gotland  to  the  coast  of 
Estland. 

Norlind2  says  that  "the  only  authentic  accounts"  of 
the  complete  freezing  of  the  Baltic  in  the  neighborhood 
of  the  Kattegat  are  in  the  years  1296,  1306,  1323,  and 
1408.  Of  these  1296  is  "much  the  most  uncertain,"  while 
1323  was  the  coldest  year  ever  recorded,  as  appears  from 
the  fact  that  horses  and  sleighs  crossed  regularly  from 
Sweden  to  Germany  on  the  ice. 

Not  only  central  Europe  and  the  shores  of  the  North 
Sea  were  marked  by  climatic  stress  during  the  four- 
teenth century,  but  Scandinavia  also  suffered.  As  Petters- 
son  puts  it : 

On  examining  the  historic  (data)  from  the  last  centuries  of 
the  Middle  Ages,  Dr.  Bull  of  Christiania  has  come  to  the  con- 
clusion that  the  decay  of  the  Norwegian  kingdom  was  not  so 
much  a  consequence  of  the  political  conditions  at  that  time,  as 
of  the  frequent  failures  of  the  harvest  so  that  corn  [wheat]  for 
bread  had  to  be  imported  from  Lubeck,  Rostock,  "Wismar  and  so 
forth.  The  Hansa  Union  undertook  the  importation  and  ob- 
tained political  power  by  its  economic  influence.  The  Norwegian 
land-owners  were  forced  to  lower  their  rents.  The  population 
decreased  and  became  impoverished.  The  revenue  sank  60  to  70 
per  cent.  Even  the  income  from  Church  property  decreased. 

2  A.  Norlind :  Einige  Bemerkungen  iiber  das  Klima  der  historischen  Zeit 
nebst  einem  Verzeichnis  mittelaltlieher  Witterungs  erscheinungen ;  Lunds 
Univ.  Arsskrift,  N.  F.,  Vol.  10,  1914,  53  pp. 


STRESS  OF  FOURTEENTH  CENTURY         101 

In  1367  corn  was  imported  from  Liibeck  to  a  value  of  one- 
half  million  kroner.  The  trade  balance  inclined  to  the  disad- 
vantage of  Norway  whose  sole  article  of  export  at  that  time  was 
dried  fish.  (The  production  of  fish  increased  enormously  in  the 
Baltic  regions  off  south  Sweden  because  of  the  same  changes 
which  were  influencing  the  lands,  but  this  did  not  benefit  Nor- 
way.) Dr.  Bull  draws  a  comparison  with  the  conditions  described 
in  the  Sagas  when  Nordland  [at  the  Arctic  Circle]  produced 
enough  corn  to  feed  the  inhabitants  of  the  country.  At  the  time 
of  Asbjorn  Selsbane  the  chieftains  in  Trondhenas  [still  farther 
north  in  latitude  69°]  grew  so  much  corn  that  they  did  not  need 
to  go  southward  to  buy  corn  unless  three  successive  years  of 
dearth  had  occurred.  The  province  of  Trondheim  exported  wheat 
to  Iceland  and  so  forth.  Probably  the  turbulent  political  state 
of  Scandinavia  at  the  end  of  the  Middle  Ages  was  in  a  great 
measure  due  to  unfavorable  climatic  conditions,  which  lowered 
the  standard  of  life,  and  not  entirely  to  misgovernment  and 
political  strife  as  has  hitherto  been  taken  for  granted. 

During  this  same  unfortunate  first  half  of  the  four- 
teenth century  England  also  suffered  from  conditions 
which,  if  sufficiently  intensified,  might  be  those  of  a  gla- 
cial period.  According  to  Thorwald  Rogers3  the  severest 
famine  ever  experienced  in  England  was  that  of  1315- 
1316,  and  the  next  worst  was  in  1321.  In  fact,  from  1308 
to  1322  great  scarcity  of  food  prevailed  most  of  the  time. 
Other  famines  of  less  severity  occurred  in  1351  and  1369. 
"The  same  cause  was  at  work  in  all  these  cases,"  says 
Rogers,  "incessant  rain,  and  cold,  stormy  summers.  It 
is  said  that  the  inclemency  of  the  seasons  affected  the 
cattle,  and  that  numbers  perished  from  disease  and 
want."  After  the  bad  harvest  of  1315  the  price  of  wheat, 
which  was  already  high,  rose  rapidly,  and  in  May,  1316, 
was  about  five  times  the  average.  For  a  year  or  more 
thereafter  it  remained  at  three  or  four  times  the  ordinary 

3  Thorwald  Rogers :  A  History  of  Agriculture  and  Prices  in  England. 


102  CLIMATIC  CHANGES 

level.  The  severity  of  the  famine  may  be  judged  from  the 
fact  that  previous  to  the  Great  War  the  most  notable 
scarcity  of  wheat  in  modern  England  and  the  highest 
relative  price  was  in  December,  1800.  At  that  time  wheat 
cost  nearly  three  times  the  usual  amount,  instead  of  five 
as  in  1316.  During  the  famine  of  the  early  fourteenth  cen- 
tury "it  is  said  that  people  were  reduced  to  subsist  upon 
roots,  upon  horses  and  dogs,  and  stories  are  told  of  even 
more  terrible  acts  by  reason  of  the  extreme  famine. ' '  The 
number  of  deaths  was  so  great  that  the  price  of  labor 
suffered  a  permanent  rise  of  at  least  10  per  cent.  There 
simply  were  not  people  enough  left  among  the  peasants 
to  do  the  work  demanded  by  the  more  prosperous  class 
who  had  not  suffered  so  much. 

After  the  famine  came  drought.  The  year  1325  appears 
to  have  been  peculiarly  dry,  and  1331,  1344,  1362,  1374, 
and  1377  were  also  dry.  In  general  these  conditions  do 
little  harm  in  England.  They  are  of  interest  chiefly  as 
showing  how  excessive  rain  and  drought  are  apt  to 
succeed  one  another. 

These  facts  regarding  northern  and  central  Europe 
during  the  fourteenth  century  are  particularly  significant 
when  compared  with  the  conclusions  which  we  have 
drawn  in  Earth  and  Sun  from  the  growth  of  trees  in 
Germany  and  from  the  distribution  of  storms.  A  careful 
study  of  all  the  facts  shows  that  we  are  dealing  with  two 
distinct  types  of  phenomena.  In  the  first  place,  the  climate 
of  central  Europe  seems  to  have  been  peculiarly  conti- 
nental during  the  fourteenth  century.  The  winters  were 
so  cold  that  the  rivers  froze,  and  the  summers  were  so 
wet  that  there  were  floods  every  other  year  or  oftener. 
This  seems  to  be  merely  an  intensification  of  the  condi- 
tions which  prevail  at  the  present  time  during  periods  of 
many  sunspots,  as  indicated  by  the  growth  of  trees  at 


STRESS  OF  FOURTEENTH  CENTURY         103 

Eberswalde  in  Germany  and  by  the  number  of  storms  in 
winter  as  compared  with  summer.  The  prevalence  of 
droughts,  especially  in  the  spring,  is  also  not  inconsistent 
with  the  existence  of  floods  at  other  seasons,  for  one  of 
the  chief  characteristics  of  a  continental  climate  is  that 
the  variations  from  one  season  to  another  are  more 
marked  than  in  oceanic  climates.  Even  the  summer 
droughts  are  typically  continental,  for  when  continental 
conditions  prevail,  the  difference  between  the  same  sea-  * 
son  in  different  years  is  extreme,  as  is  well  illustrated  in  ^/^  $&* 
Kansas.  It  must  always  be  remembered  that  what  causes 
famine  is  not  so  much  absolute  dryness  as  a  temporary 
diminution  of  the  rainfall. 

The  second  type  of  phenomena  is  peculiarly  oceanic  in 
character.  It  consists  of  two  parts,  both  of  which  are 
precisely  what  would  be  expected  if  a  highly  continental 
climate  prevailed  over  the  land.  In  the  first  place,  at  cer- 
tain times  the  cold  area  of  high  pressure,  which  is  the 
predominating  characteristic  of  a  continent  during  the 
winter,  apparently  spread  out  over  the  neighboring 
oceans.  Under  such  conditions  an  inland  sea,  such  as  the 
Baltic,  would  be  frozen,  so  that  horses  could  cross  the  ice 
even  in  the  Far  West.  In  the  second  place,  because  of  the 
unusually  high  pressure  over  the  continent,  the  baro- 
metric gradients  apparently  became  intensified.  Hence  at 
the  margin  of  the  continental  high-pressure  area  the 
winds  were  unusually  strong  and  the  storms  of  corre- 
sponding severity.  Some  of  these  storms  may  have 
passed  entirely  along  oceanic  tracks,  while  others  in- 
vaded the  borders  of  the  land,  and  gave  rise  to  the  floods 
and  to  the  wearing  away  of  the  coast  described  by 
Pettersson. 

Turning  now  to  the  east  of  Europe,  Bruckner's4  study 

<E.  Briickner:  Klimaschwankungen  seit  1700,  Vienna,  1891. 


104  CLIMATIC  CHANGES 

of  the  Caspian  Sea  shows  that  that  region  as  well  as 
western  Europe  was  subject  to  great  climatic  vicissitudes 
in  the  first  half  of  the  fourteenth  century.  In  1306-1307 
the  Caspian  Sea,  after  rising  rapidly  for  several  years, 
stood  thirty-seven  feet  above  the  present  level  and  it 
probably  rose  still  higher  during  the  succeeding  decades. 
At  least  it  remained  at  a  high  level,  for  Hamdulla,  the 
Persian,  tells  us  that  in  1325  a  place  called  Aboskun  was 
under  water.5 

Still  further  east  the  inland  lake  of  Lop  Nor  also  rose 
at  about  this  time.  According  to  a  Chinese  account  the 
Dragon  Town  on  the  shore  of  Lop  Nor  was  destroyed  by 
a  flood.  From  Himley's  translation  it  appears  that  the 
level  of  the  lake  rose  so  as  to  overwhelm  the  city  com- 
pletely. This  would  necessitate  the  expansion  of  the  lake 
to  a  point  eighty  miles  east  of  Lulan,  and  fully  fifty  from 
the  present  eastern  end  of  the  Kara  Koshun  marsh.  The 
water  would  have  to  rise  nearly,  or  quite,  to  a  strand 
which  is  now  clearly  visible  at  a  height  of  twelve  feet 
above  the  modern  lake  or  marsh. 

In  India  the  fourteenth  century  was  characterized  by 
what  appears  to  have  been  the  most  disastrous  drought 
in  all  history.  Apparently  the  decrease  in  rainfall  here 
was  as  striking  as  the  increase  in  other  parts  of  the 
world.  No  statistics  are  available  but  we  are  told  that  in 
the  great  famine  which  began  in  1344  even  the  Mogul 
emperor  was  unable  to  obtain  the  necessaries  of  life  for 
his  household.  No  rain  worth  mentioning  fell  for  years. 
In  some  places  the  famine  lasted  three  or  four  years,  and 
in  some  twelve,  and  entire  cities  were  left  without  an  in- 
habitant. In  a  later  famine,  1769-1770,  which  occurred  in 
Bengal  shortly  after  the  foundation  of  British  rule  in 

s  For  a  full  discussion  of  the  changes  in  the  Caspian  Sea  see  The  Pulse 
of  Asia,  pp.  329-358. 


STRESS  OF  FOURTEENTH  CENTURY         105 

India,  but  while  the  native  officials  were  still  in  power, 
a  third  of  the  population,  or  ten  out  of  thirty  millions, 
perished.  The  famine  in  the  first  half  of  the  fourteenth 
century  seems  to  have  been  far  worse.  These  Indian 
famines  were  apparently  due  to  weak  summer  monsoons 
caused  presumablyjjy  the  failure  of  central  Asia  to  warm 
up  as  much  as  usual.  The  heavier  snowfall,  and  the 
greater  cloudiness  of  the  summer  there,  which  probably 
accompanied  increased  storminess,  may  have  been  the 
reason. 

The  New  World  as  well  as  the  Old  appears  to  have 
been  in  a  state  of  climatic  stress  during  the  first  half  of 
the  fourteenth  century.  According  to  Pettersson,  Green- 
land furnishes  an  example  of  this.  At  first  the  inhabitants 
of  that  northland  were  fairly  prosperous  and  were  able 
to  approach  from  Iceland  without  much  hindrance  from 
the  ice.  Today  the  North  Atlantic  Ocean  northeast  of 
Iceland  is  full  of  drift  ice  much  of  the  time.  The  border 
of  the  ice  varies  from  season  to  season,  but  in  general  it 
extends  westward  from  Iceland  not  far  from  the  Arctic 
circle  and  then  follows  the  coast  of  Greenland  south- 
ward to  Cape  Farewell  at  the  southern  tip  and  around  to 
the  western  side  for  fifty  miles  or  more.  Except  under 
exceptional  circumstances  a  ship  cannot  approach  the 
coast  until  well  northward  on  the  comparatively  ice-free 
west  coast.  In  the  old  Sagas,  however,  nothing  is  said  of 
ice  in  this  region.  The  route  from  Iceland  to  Greenland 
is  carefully  described.  In  the  earliest  times  it  went  from 
Iceland  a  trifle  north  of  west  so  as  to  approach  the  coast 
of  Greenland  after  as  short  an  ocean  passage  as  possible. 
Then  it  went  down  the  coast  in  a  region  where  approach 
is  now  practically  impossible  because  of  the  ice.  At  that 
time  this  coast  was  icy  close  to  the  shore,  but  there  is  no 
sign  that  navigation  was  rendered  difficult  as  is  now  the 


106  CLIMATIC  CHANGES 

case.  Today  no  navigator  would  think  of  keeping  close 
inland.  The  old  route  also  went  north  of  the  island  on 
which  Cape  Farewell  is  located,  although  the  narrow 
channel  between  the  island  and  the  mainland  is  now  so 
blocked  with  ice  that  no  modern  vessel  has  ever  pene- 
trated it.  By  the  thirteenth  century,  however,  there  ap- 
pears to  have  been  a  change.  In  the  Kungaspegel  or 
Kings'  Mirror,  written  at  that  time,  navigators  are 
warned  not  to  make  the  east  coast  too  soon  on  account 
of  ice,  but  no  new  route  is  recommended  in  the  neighbor- 
hood of  Cape  Farewell  or  elsewhere.  Finally,  however, 
at  the  end  of  the  fourteenth  century,  nearly  150  years 
after  the  Kungaspegel,  the  old  sailing  route  was  aban- 
doned, and  ships  from  Iceland  sailed  directly  southwest 
to  avoid  the  ice.  As  Pettersson  says : 

...  At  the  end  of  the  thirteenth  and  the  beginning  of  the 
fourteenth  century  the  European  civilization  in  Greenland  was 
wiped  out  by  an  invasion  of  the  aboriginal  population.  The  col- 
onists in  the  Vesterbygd  were  driven  from  their  homes  and 
probably  migrated  to  America  leaving  behind  their  cattle  in  the 
fields.  So  they  were  found  by  Ivar  Bardsson,  steward  to  the 
Bishop  of  Gardar,  in  his  official  journey  thither  in  1342. 

The  Eskimo  invasion  must  not  be  regarded  as  a  common  raid. 
It  was  the  transmigration  of  a  people,  and  like  other  big  move- 
ments of  this  kind  [was]  impelled  by  altered  conditions  of 
nature,  in  this  case  the  alterations  of  climate  caused  by  [or 
which  caused?]  the  advance  of  the  ice.  For  their  hunting  and 
fishing  the  Eskimos  require  an  at  least  partially  open  arctic 
sea.  The  seal,  their  principal  prey,  cannot  live  where  the  surface 
of  the  sea  is  entirely  frozen  over.  The  cause  of  the  favorable 
conditions  in  the  Viking-age  was,  according  to  my  hypothesis, 
that  the  ice  then  melted  at  a  higher  latitude  in  the  arctic  seas. 

The  Eskimos  then  lived  further  north  in  Greenland  and 
North  America.  When  the  climate  deteriorated  and  the  sea  which 
gave  them  their  living  was  closed  by  ice  the  Eskimos  had  to  find 


STRESS  OF  FOURTEENTH  CENTURY         107 

a  more  suitable  neighborhood.  This  they  found  in  the  land 
colonized  by  the  Norsemen  whom  they  attacked  and  finally 
annihilated. 

Finally,  far  to  the  south  in  Yucatan  the  ancient  Maya 
civilization  made  its  last  flickering  effort  at  about  this 
time.  Not  much  is  known  of  this  but  in  earlier  periods 
the  history  of  the  Mayas  seems  to  have  agreed  quite 
closely  with  the  fluctuations  in  climate.8  Among  the 
Mayas,  as  we  have  seen,  relatively  dry  periods  were  the 
times  of  greatest  progress. 

Let  us  turn  now  to  Fig.  3  once  more  and  compare  the 
climatic  conditions  of  the  fourteenth  century  with  those 
of  periods  of  increasing  rainfall.  Southern  England, 
Ireland,  and  Scandinavia,  where  the  crops  were  ruined 
by  extensive  rain  and  storms  in  summer,  are  places 
where  storminess  and  rainfall  now  increase  when  sun- 
spots  are  numerous.  Central  Europe  and  the  coasts  of  the 
North  Sea,  where  flood  and  drought  alternated,  are  re- 
gions which  now  have  relatively  less  rain  when  sunspots 
increase  than  when  they  diminish.  However,  as  appears 
from  the  trees  measured  by  Douglass,  the  winters  become 
more  continental  and  hence  cooler,  thus  corresponding  to 
the  cold  winters  of  the  fourteenth  century  when  people 
walked  on  the  ice  from  Scandinavia  to  Denmark.  When 
such  high  pressure  prevails  in  the  winter,  the  total  rain- 
fall is  diminished,  but  nevertheless  the  storms  are  more 
severe  than  usual,  especially  in  the  spring.  In  south- 
eastern Europe,  the  part  of  the  area  whence  the  Caspian 
derives  its  water,  appears  to  have  less  rainfall  during 
times  of  increasing  sunspots  than  when  sunspots  are  few, 
but  in  an  equally  large  area  to  the  south,  where  the  moun- 

«  S.  Q.  Morley:  The  Inscriptions  at  Copan;  Carnegie  Inst.  of  Wash.,  No. 
219,  1920. 

Ellsworth  Huntington:  The  Red  Man's  Continent,  1919. 


108  CLIMATIC  CHANGES 

tains  are  higher  and  the  run-off  of  the  rain  is  more  rapid, 
the  reverse  is  the  case.  This  seems  to  mean  that  a  slight 
diminution  in  the  water  poured  in  by  the  Volga  would 
be  more  than  compensated  by  the  water  derived  from 
Persia  and  from  the  Oxus  and  Jaxartes  rivers,  which  in 
the  fourteenth  century  appear  to  have  filled  the  Sea  of 
Aral  and  overflowed  in  a  large  stream  to  the  Caspian. 
Still  farther  east  in  central  Asia,  so  far  as  the  records  go, 
most  of  the  country  receives  more  rain  when  sunspots 
are  many  than  when  they  are  few,  which  would  agree 
with  what  happened  when  the  Dragon  Town  was  inun- 
dated. In  India,  on  the  contrary,  there  is  a  large  area 
where  the  rainfall  diminishes  at  times  of  many  sunspots, 
thus  agreeing  with  the  terrible  famine  from  which  the 
Moguls  suffered  so  severely.  In  the  western  hemisphere, 
Greenland,  Arizona,  and  California  are  all  parts  of  the 
area  where  the  rain  increases  with  many  sunspots,  while 
Yucatan  seems  to  lie  in  an  area  of  the  opposite  type.  Thus 
all  the  evidence  seems  to  show  that  at  times  of  climatic 
stress,  such  as  the  fourteenth  century,  the  conditions 
are  essentially  the  same  as  those  which  now  prevail  at 
times  of  increasing  sunspots. 

As  to  the  number  of  sunspots,  there  is  little  evidence 
previous  to  about  1750.  Yet  that  little  is  both  interesting 
and  important.  Although  sunspots  have  been  observed 
with  care  in  Europe  only  a  little  more  than  three  cen- 
turies, the  Chinese  have  records  which  go  back  nearly  to 
the  beginning  of  the  Christian  era.  Of  course  the  records 
are  far  from  perfect,  for  the  work  was  done  by  indi- 
viduals and  not  by  any  great  organization  which  con- 
tinued the  same  methods  from  generation  to  generation. 
The  mere  fact  that  a  good  observer  happened  to  use  his 
smoked  glass  to  advantage  may  cause  a  particular  period 
to  appear  to  have  an  unusual  number  of  spots.  On  the 


STRESS  OF  FOURTEENTH  CENTURY         109 

other  hand,  the  fact  that  such  an  observer  finds  spots 
at  some  times  and  not  at  others  tends  to  give  a  valuable 
check  on  his  results,  as  does  the  comparison  of  one 
observer's  work  with  that  of  another.  Hence,  in  spite  of 
many  and  obvious  defects,  most  students  of  the  problem 
agree  that  the  Chinese  record  possesses  much  value,  and 
that  for  a  thousand  years  or  more  it  gives  a  fairly  true 
idea  of  the  general  aspect  of  the  sun.  In  the  Chinese 
records  the  years  with  many  spots  fall  in  groups,  as 
would  be  expected,  and  are  sometimes  separated  by  long 
intervals.  Certain  centuries  appear  to  have  been  marked  ~  JA 
by  unusual  spottedness.  The  most  conspicuous  of  the§fiu— -^-j 
is  the  fourteenth,  when  the  years  VI370  to  1385  were  par- 
ticularly noteworthy,  for  spots  large  enough  to  be  visible 
to  the  naked  eye  covered  the  sun  much  of  the  time.  Hence 
Wolf,7  who  has  made  an  exhaustive  study  of  the  matter, 
concludes  that  there  was  an  absolute  maximum  of  spots 
about  1372.  While  this  date  is  avowedly  open  to  question, 
the  great  abundance  of  sunspots  at  that  time  makes  it 
probable  that  it  cannot  be  far  wrong.  If  this  is  so,  it 
seems  that  the  great  climatic  disturbances  of  which  we 
have  seen  evidence  in  the  fourteenth  century  occurred  at 
a  time  when  sunspots  were  increasing,  or  at  least  when 
solar  activity  was  under  some  profoundly  disturbing  in- 
fluence. Thus  the  evidence  seems  to  show  not  merely  that 
the  climate  of  historic  times  has  been  subject  to  im- 
portant pulsations,  but  that  those  pulsations  were  mag- 
nifications of  the  little  climatic  changes  which  now  take 
place  in  sunspot  cycles.  The  past  and  the  present  are 
apparently  a  unit  except  as  to  the  intensity  of  the 
changes. 

?  See  summary  of  Wolf's  work  with  additional  information  by  H.  Fritz; 
Zurich  Vierteljahrschrift,  Vol.  38,  1893,  pp.  77-107. 


CHAPTER  VII 

GLACIATION  ACCORDING  TO  THE  SOLAR- 
CYCLONIC  HYPOTHESIS1 

THE  remarkable  phenomena  of  glacial  periods 
afford  perhaps  the  best  available  test  to  which 
any  climatic  hypothesis  can  be  subjected.  In  this 
chapter  and  the  two  that  follow,  we  shall  apply  this  test. 
Since  much  more  is  known  about  the  recent  Great  Ice 
Age,  or  Pleistocene  glaciation,  than  about  the  more 
ancient  glaciations,  the  problems  of  the  Pleistocene  will 
receive  especial  attention.  In  the  present  chapter  the 
oncoming  of  glaciation  and  the  subsequent  disappear- 
ance of  the  ice  will  be  outlined  in  the  light  of  what  would 
be  expected  according  to  the  solar-cyclonic  hypothesis. 
Then  in  the  next  chapter  several  problems  of  especial 
climatic  significance  will  be  considered,  such  as  the  locali- 
zation of  ice  sheets,  the  succession  of  severe  glacial  and 
mild  inter-glacial  epochs,  the  sudden  commencement  of 
glaciation  and  the  peculiar  variations  in  the  height  of  the 
snow  line.  Other  topics  to  be  considered  are  the  occur- 
rence of  pluvial  or  rainy  climates  in  non-glaciated  re- 
gions, and  glaciation  near  sea  level  in  subtropical 
latitudes  during  the  Permian  and  Proterozoic.  Then  in 
Chapter  IX  we  shall  consider  the  development  and  dis- 
tribution of  the  remarkable  deposits  of  wind-blown  ma- 
terial known  as  loess. 
Facts  not  considered  at  the  time  of  framing  an  hypothe- 

i  This  chapter  is  an  amplification  and  revision  of  the  sketch  of  the  glacial 
period  contained  in  The  Solar  Hypothesis  of  Climatic  Changes;  Bull.  Geol. 
Soc.  Am.,  Vol.  25,  1914. 


THE  GLACIAL  PERIOD  111 

sis  are  especially  significant  in  testing  it.  In  this  particu- 
lar case,  the  cyclonic  hypothesis  was  framed  to  explain 
the  historic  changes  of  climate  revealed  by  a  study  of 
ruins,  tree  rings,  and  the  terraces  of  streams  and  lakes, 
without  special  thought  of  glaciation  or  other  geologic 
changes.  Indeed,  the  hypothesis  had  reached  nearly  its 
present  form  before  much  attention  was  given  to  geo- 
logical phases  of  the  problem.  Nevertheless,  it  appears 
to  meet  even  this  severe  test. 

According  to  the  solar-cyclonic  hypothesis,  the  Pleisto- 
cene glacial  period  was  inaugurated  at  a  time  when  cer- 
tain terrestrial  conditions  tended  to  make  the  earth 
especially  favorable  for  glaciation.  How  these  conditions 
arose  will  be  considered  later.  Here  it  is  enough  to  state 
what  they  were.  Chief  among  them  was  the  fact  that  the 
continents  stood  unusually  high  and  were  unusually 
large.  This,  however,  was  not  the  primary  cause  of  gla- 
ciation, for  many  of  the  areas  which  were  soon  to  be 
glaciated  were  little  above  sea  level.  For  example,  it 
seems  clear  that  New  England  stood  less  than  a  thousand 
feet  higher  than  now.  Indeed,  Salisbury2  estimates  that 
eastern  North  America  in  general  stood  not  more  than 
a  few  hundred  feet  higher  than  now,  and  W.  B.  Wright3 
reaches  the  same  conclusion  in  respect  to  the  British 
Isles.  Nevertheless,  widespread  lands,  even  if  they  are 
not  all  high,  lead  to  climatic  conditions  which  favor 
glaciation.1"  For  example,  enlarged  continents  cause  low 
temperature  in  high  latitudes  because  they  interfere  with 
the  ocean  currents  that  carry  heat  polewards.  Such  con- 
tinents also  cause  relatively  cold  winters,  for  lands  cool 
much  sooner  than  does  the  ocean.  Another  result  is  a 

2E.  D.  Salisbury:  Physical  Geography  of  the  Pleistocene,  in  Outlines  of 
Geologic  History,  by  Willis,  Salisbury,  and  others,  1910,  p.  265. 
s  The  Quaternary  Ice  Age,  1914,  p.  364. 


112  CLIMATIC  CHANGES 

diminution  of  water  vapor,  not  only  because  cold  air 
cannot  hold  much  vapor,  but  also  because  the  oceanic 
area  from  which  evaporation  takes  place  is  reduced  by 
the  emergence  of  the  continents.  Again,  when  the  conti- 
nents are  extensive  the  amount  of  carbonic  acid  gas  in 
the  atmosphere  probably  decreases,  for  the  augmented 
erosion  due  to  uplift  exposes  much  igneous  rock  to  the 
air,  and  weathering  consumes  the  atmospheric  carbon 
dioxide.  When  the  supply  of  water  vapor  and  of  atmos- 
pheric carbon  dioxide  is  small,  an  extreme  type  of  climate 
f.  S  usually  prevails.  The  combined  result  of  all  these  condi- 
tions is  that  continental  emergence  causes  the  climate  to 
be  somewhat  cool  and  to  be  marked  by  relatively  great 
contrasts  from  season  to  season  and  from  latitude  to 
latitude. 

When  the  terrestrial  conditions  thus  permitted  glacia- 
tion,  unusual  solar  activity  is  supposed  to  have  greatly 
increased  the  number  and  severity  of  storms  and  to  have 
altered  their  location,  just  as  now  happens  at  times  of 
many  sunspots.  If  such  a  change  in  storminess  had  oc- 
curred when  terrestrial  conditions  were  unfavorable  for 
glaciation,  as,  for  example,  when  the  lands  were  low  and 
there  were  widespread  epicontinental  seas  in  middle  and 
high  latitudes,  glaciation  might  not  have  resulted.  In  the 
Pleistocene,  however,  terrestrial  conditions  permitted 
glaciation,  and  therefore  the  supposed  increase  in  stormi- 
ness caused  great  ice  sheets. 

The  conditions  which  prevail  at  times  of  increased 
storminess  have  been  discussed  in  detail  in  Earth  and 
Sun.  Those  which  apparently  brought  on  glaciation  seem 
to  have  acted  as  follows :  In  the  first  place  the  storminess 
lowered  the  temperature  of  the  earth's  surface  in  several 
ways.  The  most  important  of  these  was  the  rapid  upward  n 
convection  in  the  centers  of  cyclonic  storms  whereby 


THE  GLACIAL  PERIOD  113 

abundant  heat  was  carried  to  high  levels  where  most  of  it 
was  radiated  away  into  space.  The  marked  increase  in 
the  number  of  tropical  cyclones  which  accompanies  in- 
creased solar  activity  was  probably  important  in  this 
respect.  Such  cyclones  carry  vast  quantities  of  heat  and 
moisture  out  of  the  tropics.  The  moisture,  to  be  sure, 
liberates  heat  upon  condensing,  but  as  condensation 
occurs  above  the  earth's  surface,  much  of  the  heat 
escapes  into  space.  Another  reason  for  low  temperature 
was  that  under  the  influence  of  the  supposedly  numerous 
storms  of  Pleistocene  times  evaporation  over  the  oceans 

.ust  have  increased.  This  is  largely  because  the  velocity 
of  the  winds  is  relatively  great  when  storms  are  strong 
and  such  winds  are  powerful  agents  of  evaporation.  But 
evaporation  requires  heat,  and  hence  the  strong  winds 
lower  the  temperature.311 

The  second  great  condition  which  enabled  increased 
storminess  to  bring  on  glaciation  was  the  location  of  the 
storm  tracks.  Kullmer's  maps,  as  illustrated  in  Fig.  2, 
suggest  that  a  great  increase  in  solar  activity,  such  as  is 
postulated  in  the  Pleistocene,  might  shift  the  main  storm 
track  poleward  even  more  than  it  is  shifted  by  the  milder 
solar  changes  during  the  twelve-year  sunspot  cycle.  If 
this  is  so,  the  main  track  would  tend  to  cross  North 
America  through  the  middle  of  Canada  instead  of  near 
the  southern  border.  Thus  there  would  be  an  increase  in 
precipitation  in  about  the  latitude  of  the  Keewatin  and 
Labradorean  centers  of  glaciation.  From  what  is  known 
of  storm  tracks  in  Europe,  the  main  increase  in  the  in- 
tensity of  storms  would  probably  center  in  Scandinavia. 
Fig.  3  in  Chapter  V  bears  this  out.  That  figure,  it  will  be 
recalled,  shows  what  happens  to  precipitation  when  solar 


fuller  discussion  of  climatic  controls  see  S.  S.  Visher:    Seventy 
Laws  of  Climate,  Annals  Assoc.  Am.  Geographers,  1922. 


114  CLIMATIC  CHANGES 

activity  is  increasing.  A  high  rate  of  precipitation  is 
especially  marked  in  the  boreal  storm  track,  that  is,  in 
the  northern  United  States,  southern  Canada,  and  north- 
western Europe. 

Another  important  condition  in  bringing  on  glaciation 
would  be  the  fact  that  when  storms  are  numerous  the 
total  precipitation  appears  to  increase  in  spite  of  the 
slightly  lower  temperature.  This  is  largely  because  of  the 
greater  evaporation.  The  excessive  evaporation  arises 
partly  from  the  rapidity  of  the  winds,  as  already  stated, 
and  partly  from  the  fact  that  in  areas  where  the  air  is 
clear  the  sun  would  presumably  be  able  to  act  more  effec- 
tively than  now.  It  would  do  so  because  at  times  of  abun- 
dant sunspots  the  sun  in  our  own  day  has  a  higher  solar 
constant  than  at  times  of  milder  activity.  Our  whole 
hypothesis  is  based  on  the  supposition  that  what  now 
happens  at  times  of  many  sunspots  was  intensified  in 
glacial  periods. 

A  fourth  condition  which  would  cause  glaciation  to 
result  from  great  solar  activity  would  be  the  fact  that 
the  portion  of  the  yearly  precipitation  falling  as  snow 
would  increase,  while  the  proportion  of  rain  would  dimin- 
ish in  the  main  storm  track.  This  would  arise  partly  be- 
cause the  storms  would  be  located  farther  north  than 
now,  and  partly  because  of  the  diminution  in  temperature 
due  to  the  increased  convection.  The  snow  in  itself  would 
still  further  lower  the  temperature,  for  snow  is  an  excel- 
lent reflector  of  sunlight.  The  increased  cloudiness  which 
would  accompany  the  more  abundant  storms  would  also 
cause  an  unusually  great  reflection  of  the  sunlight  and 
still  further  lower  the  temperature.  Thus  at  times  of 
many  sunspots  a  strong  tendency  toward  the  accumula- 
tion of  snow  would  arise  from  the  rapid  convection  and 
consequent  low  temperature,  from  the  northern  location 


THE  GLACIAL  PERIOD  115 

of  storms,  from  the  increased  evaporation  and  precipita- 
tion, from  the  larger  percentage  of  snowy  rather  than 
rainy  precipitation,  and  from  the  great  loss  of  heat  due 
to  reflection  from  clouds  and  snow. 

If  events  at  the  beginning  of  the  last  glacial  period 
took  place  in  accordance  with  the  cyclonic  hypothesis,  as 
outlined  above,  one  of  the  inevitable  results  would  be  the 
production  of  snowfields.  The  places  where  snow  would 
accumulate  in  special  quantities  would  be  central  Canada, 
the  Labrador  plateau,  and  Scandinavia,  as  well  as  cer- 
tain mountain  regions.  As  soon  as  a  snowfield  became 
somewhat  extensive,  it  would  begin  to  produce  striking  • 
climatic  alterations  in  addition  to  those  to  which  it  owed 
its  origin.4  For  example,  within  a  snowfield  the  summers 
remain  relatively  cold.  Hence  such  a  field  is  likely  to  be  / 
an  area  of  high  pressure  at  all  seasons.  The  fact  that  the 
snowfield^is  always  a  place  of  relatively  high  pressure 
results  in  outblowing  surface  winds  except  when  these 
are  temporarily  overcome  by  the  passage  of  strong  cy- 
clonic storms.  The  storms,  however,  tend  to  be  concen- 
trated near  the  margins  of  the  ice  throughout  the  year 
instead  of  following  different  paths  in  each  of  the  four 
seasons.  This  is  partly  because  cyclonic  lows  always 
avoid  places  of  high  pressure  and  are  thus  pushed  out 
of  the  areas  where  permanent  snow  has  accumulated. 
On  the  other  hand,  at  times  of  many  sunspots,  as  Kull- 
mer  has  shown,  the  main  storm  track  tends  to  be  drawn 

4  Many  of  these  alterations  are  implied  or  discussed  in  the  following 
papers : 

1.  F.  W.  Harmer:  Influence  of  Winds  upon  the  Climate  of  the  Pleisto- 
cene; Quart.  Jour.  Geol.  Soc.,  Vol.  57,  1901,  p.  405. 

2.  C.  E.  P.  Brooks:  Meteorological  Conditions  of  an  Ice  Sheet;  Quart. 
Jour.  Eoyal  Meteorol.  Soc.,  Vol.  40,  1914,  pp.  53-70,  and  The  Evolution  of 
Climate  in  Northwest  Europe;  op.  tit.,  Vol.  47,  1921,  pp.  173-194. 

3.  W.  H.  Hobbs:  The  Bole  of  the  Glacial  Anticyclone  in  the  Air  Circu- 
lation of  the  Globe;  Proc.  Am.  Phil.  Soc.,  Vol.  54,  1915,  pp.  185-225. 


116  CLIMATIC  CHANGES 

poleward,  perhaps  by  electrical  conditions.  Hence  when 
a  snowfield  is  present  in  the  north,  the  lows,  instead  of 
migrating  much  farther  north  in  summer  than  in  winter, 
as  they  now  do,  would  merely  crowd  on  to  the  snowfield 
a  little  farther  in  summer  than  in  winter.  Thus  the  heavy 
precipitation  which  is  usual  in  humid  climates  near  the 
centers  of  lows  would  take  place  near  the  advancing 
margin  of  the  snowfield  and  cause  the  field  to  expand 
still  farther  southward. 

The  tendency  toward  the  accumulation  of  snow  on  the 
margins  of  the  snowfields  would  be  intensified  not  only 
by  the  actual  storms  themselves,  but  by  other  conditions. 
For  example,  the  coldness  of  the  snow  would  tend  to 
cause  prompt  condensation  of  the  moisture  brought  by 
the  winds  that  blow  toward  the  storm  centers  from  low 
latitudes.  Again,  in  spite  of  the  general  dryness  of  the 
'air  over  a  snowfield,  the  lower  air  contains  some  moisture 
due  to  evaporation  from  the  snow  by  day  during  the 
clear  sunny  weather  of  anti-cyclones  or  highs.  Where  this 
is  sufficient,  the  cold  surface  of  the  snowfields  tends  to 
produce  a  frozen  fog  whenever  the  snowfield  is  cooled 
by  radiation,  as  happens  at  night  and  during  the  passage 
of  highs.  Such  a  frozen  fog  is  an  effective  reflector  of 
solar  radiation.  Moreover,  because  ice  has  only  half  the 
specific  heat  of  water,  and  is  much  more  transparent  to 
heat,  such  a  "radiation  fog"  composed  of  ice  crystals  is 
a  much  less  effective  retainer  of  heat  than  clouds  or  fog 
made  of  unfrozen  water  particles.  Shallow  fogs  of  this 
type  are  described  by  several  polar  expeditions.  They 
clearly  retard  the  melting  of  the  snow  and  thus  help  the 
icefield  to  grow. 

For  all  these  reasons,  so  long  as  storminess  remained 
great,  the  Pleistocene  snowfields,  according  to  the  solar 
hypothesis,  must  have  deepened  and  expanded.  In  due 


THE  GLACIAL  PERIOD  117 

time  some  of  the  snow  was  converted  into  glacial  ice. 
When  that  occurred,  the  growth  of  the  snowfield  as  well 
as  of  the  ice  cap  must  have  been  accelerated  by  glacial 
movement.  Under  such  circumstances,  as  the  ice  crowded 
southward  toward  the  source  of  the  moisture  by  which  it 
grew,  the  area  of  high  pressure  produced  by  its  low_ 
temperature  would  expand.  This  would  force  the  storm 
track  southward  in  spite  of  the  contrary  tendency  due  to 
the  sun.  When  the  ice  sheet  had  become  very  extensive, 
the  track  would  be  crowded  relatively  near  to  the  north- 
ern margin  of  the  trade-wind  belt.  Indeed,  the  Pleisto- 
cene ice  sheets,  at  the  time  of  their  maximum  extension,  A  j...  $  ^ 
reached  almost  as  far  south  as  the  latitude  now  marking 
the  northern  limit  of  the  trade-wind  belt  in  summer. 
As  the  storm  track  with  its  frequent  low  pressure  and  the 
subtropical  belt  with  its  high  pressure  were  forced  nearer 
and  nearer  together,  the  barometric  gradient  between 
the  two  presumably  became  greater,  winds  became 
stronger,  and  the  storms  more  intense. 

This  zonal  crowding  would  be  of  special  importance  in 
summer,  at  which  time  it  would  also  be  most  pronounced. 
In  the  first  place,  the  storms  would  be  crowded  far  upon 
the  ice  cap  which  would  then  be  protected  from  the  sun 
by  a  cover  of  fog  and  cloud  more  fully  than  at  any  other 
season.  Furthermore,  the  close  approach  of  the  trade- 
wind  belt  to  the  storm  belt  would  result  in  a  great  in-  rjj^ 
crease  in  the  amount  of  moisture  drawn  from  the  belt  of 
evaporation  which  the  trade  winds  dominate.  In  the 
trade-wind  belt,  clear  skies  and  high  temperature  make 
evaporation  especially  rapid.  Indeed,  in  spite  of  the  vast 
deserts  it  is  probable  that  more  than  three-fourths  of  the 
total  evaporation  now  taking  place  on  the  earth  occurs 
in  the  belt  of  trades,  an  area  which  includes  about  one- 
half  of  the  earth's  surface. 


118  CLIMATIC  CHANGES 

The  agency  which  could  produce  this  increased  draw- 
ing northward  of  moisture  from  the  trade-wind  belt 
would  be  the  winds  blowing  into  the  lows.  According  to 
the  cyclonic  hypothesis,  many  of  these  lows  would  be  so 
strong  that  they  would  temporarily  break  down  the  sub- 
tropical belt  of  high  pressure  which  now  usually  prevails 
between  the  trades  and  the  zone  of  westerly  winds.  This 
belt  is  even  now  often  broken  by  tropical  cyclones.  If  the 
storms  of  more  northerly  regions  temporarily  destroyed 
the  subtropical  high-pressure  belt,  even  though  they  still 
remained  on  its  northern  side,  they  would  divert  part  of 
the  trade  winds.  Hence  the  air  which  now  is  carried 
obliquely  equatorward  by  those  winds  would  be  carried 
spirally  northward  into  the  cyclonic  lows.  Precipitation 
in  the  storm  track  on  the  margin  of  the  relatively  cold  ice 
sheet  would  thus  be  much  increased,  for  most  winds  from 
low  latitudes  carry  abundant  moisture.  Such  a  diversion 
of  moisture  from  low  latitudes  probably  explains  the 
deficiency  of  precipitation  along  the  heat  equator  at 
times  of  solar  activity,  as  shown  in  Fig.  3.  Taken  as  a 
whole,  the  summer  conditions,  according  to  the  cyclonic 
hypothesis,  would  be  such  that  increased  evaporation  in 
low  latitudes  would  cooperate  with  increased  storminess, 
cloudiness,  and  fog  in  higher  latitudes  to  preserve  and 
increase  the  accumulation  of  ice  upon  the  borders  of  the 
ice  sheet.  The  greater  the  storminess,  the  more  this  would 
be  true  and  the  more  the  ice  sheet  would  be  able  to  hold 
its  own  against  melting  in  summer.  Such  a  combination 
of  precipitation  and  of  protection  from  the  sun  is  espe- 
cially important  if  an  ice  sheet  is  to  grow. 

The  meteorologist  needs  no  geologic  evidence  that  the 
storm  track  was  shoved  equatorward  by  the  growth  of 
the  ice  sheet,  for  he  observes  a  similar  shifting  whenever 
a  winter's  snow  cap  occupies  part  of  the  normal  storm 


THE  GLACIAL  PERIOD  119 

tract.  The  geologist,  however,  may  welcome  geologic 
evidence  that  such  an  extreme  shift  of  the  storm  track 
actually  occurred  during  the  Pleistocene.  Harmer,  in 
1901,  first  pointed  out  the  evidence  which  was  repeated 
with  approval  by  Wright  of  the  Ireland  Geological  Sur- 
vey in  1914.5  According  to  these  authorities,  numerous 
boulders  of  a  distinctive  chalk  were  deposited  by  Pleisto- 
cene icebergs  along  the  coast  of  Ireland.  Their  distribu- 
tion shows  that  at  the  time  of  maximum  glaciation  the 
strong  winds  along  the  south  coast  of  Ireland  were  from 
the  northeast  while  today  they  are  from  the  southwest. 
Such  a  reversal  could  apparently  be  produced  only  by  a 
southward  shift  of  the  center  of  the  main  storm  track 
from  its  present  position  in  northern  Ireland,  Scotland, 
and  Norway  to  a  position  across  northern  France,  central 
Germany,  and  middle  Russia.  This  would  mean  that  while 
now  the  centers  of  the  lows  commonly  move  northeast- 
ward a  short  distance  north  of  southern  Ireland,  they 
formerly  moved  eastward  a  short  distance  south  of  Ire- 
land. It  will  be  recalled  that  in  the  northern  hemisphere 
the  winds  spiral  into  a  low  counter-clockwise  and  that 
they  are  strongest  near  the  center.  When  the  centers  pass 
not  far  north  of  a  given  point,  the  strong  winds  therefore 
blow  from  the  west  or  southwest,  while  when  the  centers 
pass  just  south  of  that  point,  the  strong  winds  come  from 
the  east  or  northeast. 

In  addition  to  the  consequences  of  the  crowding  of  the 
storm  track  toward  the  trade-wind  belt,  several  other 
conditions  presumably  operated  to  favor  the  growth  of 
the  ice  sheet.  For  example,  the  lowering  of  the  sea  level 
by  the  removal  of  water  to  form  the  snowfields  and 
glaciers  interfered  with  warm  currents.  It  also  increased 
the  rate  of  erosion,  for  it  was  equivalent  to  an  uplift  of 

s  W.  B.  Wright :  The  Quaternary  Ice  Age,  1914,  p.  100. 


120  CLIMATIC  CHANGES 

all  the  land.  One  consequence  of  erosion  and  weathering 
was  presumably  a  diminution  of  the  carbon  dioxide  in 
the  atmosphere,  for  although  the  ice  covered  perhaps  a 
tenth  of  the  lands  and  interfered  with  carbonation  to  that 
extent,  the  removal  of  large  quantities  of  soil  by  acceler- 
ated erosion  on  the  other  nine-tenths  perhaps  more  than 
counterbalanced  the  protective  effect  of  the  ice.  At  the 
same  time,  the  general  lowering  of  the  temperature  of  the 
ocean  as  well  as  the  lands  increased  the  ocean's  capacity 
for  carbon  dioxide  and  thus  facilitated  absorption.  At  a 
temperature  of  50° F.  water  absorbs  32  per  cent  more 
carbon  dioxide  than  at  68°.  The  high  waves  produced  by 
the  severe  storms  must  have  had  a  similar  effect  on  a 
small  scale.  Thus  the  percentage  of  carbon  dioxide  in  the 
atmosphere  was  presumably  diminished.  Of  less  signifi- 
cance than  these  changes  in  the  lands  and  the  air,  but 
perhaps  not  negligible,  was  the  increased  salinity  of  the 
ocean  which  accompanied  the  removal  of  water  to  form 
snow,  and  the  increase  of  the  dissolved  mineral  load  of 
the  rejuvenated  streams.  Increased  salinity  slows  up  the 
deep-sea  circulation,  as  we  shall  see  in  a  later  chapter. 
This  increases  the  contrasts  from  zone  to  zone. 

At  times  of  great  solar  activity  the  agencies  mentioned 
above  would  apparently  cooperate  to  cause  an  advance  of 
ice  sheets  into  lower  latitudes.  The  degree  of  solar  activ- 
ity would  have  much  to  do  with  the  final  extent  of  the  ice 
sheets.  Nevertheless,  certain  terrestrial  conditions  would 
tend  to  set  limits  beyond  which  the  ice  would  not  greatly 
advance  unless  the  storminess  were  extraordinarily 
severe.  The  most  obvious  of  these  conditions  is  the  loca- 
tion of  oceans  and  of  deserts  or  semi-arid  regions.  The 
southwestward  advance  of  the  European  ice  sheet  and 
the  southeastward  advance  of  the  Labradorean  sheet  in 
America  were  stopped  by  the  Atlantic.  The  semi-aridity 


THE  GLACIAL  PERIOD  121 

of  the  Great  Plains,  produced  by  their  position  in  the  lee 
of  the  Eocky  Mountains,  stopped  the  advance  of  the 
Keewatin  ice  sheet  toward  the  southwest.  The  advance  of 
the  European  ice  sheet  southeast  seems  to  have  been 
stopped  for  similar  reasons.  The  cessation  of  the  advance 
would  be  brought  about  in  such  an  area  not  alone  by  the 
light  precipitation  and  abundant  sunshine,  but  by  the 
dryness  of  the  air,  and  also  by  the  power  of  dust  to  ab- 
sorb the  sun's  heat.  Much  dust  would  presumably  be 
drawn  in  from  the  dry  regions  by  passing  cyclonic  storms 
and  would  be  scattered  over  the  ice. 

The  advance  of  the  ice  is  also  slowed  up  by  a  rugged 
topography,  as  among  the  Appalachians  in  northern 
Pennsylvania.  Such  a  topography  besides  opposing  a 
physical  obstruction  to  the  movement  of  the  ice  provides 
bare  south-facing  slopes  which  the  sun  warms  effectively. 
Such  warm  slopes  are  unfavorable  to  glacial  advance. 
The  rugged  topography  was  perhaps  quite  as  effective  as 
the  altitude  of  the  Appalachians  in  causing  the  conspicu- 
ous northward  dent  in  the  glacial  margin  in  Pennsyl- 
vania. Where  glaciers  lie  in  mountain  valleys  the  advance 
beyond  a  certain  point  is  often  interfered  with  by  the 
deployment  of  the  ice  at  the  mouths  of  gorges.  Evapora- 
tion and  melting  are  more  rapid  where  a  glacier  is  broad 
and  thin  than  where  it  is  narrow  and  thick,  as  in  a  gorge. 
Again,  where  the  topography  or  the  location  of  oceans  or 
dry  areas  causes  the  glacial  lobes  to  be  long  and  narrow, 
the  elongation  of  the  lobe  is  apparently  checked  in  sev- 
eral ways.  Toward  the  end  of  the  lobe,  melting  and 
evaporation  increase  rapidly  because  the  planetary 
westerly  winds  are  more  likely  to  overcome  the  glacial 
winds  and  sweep  across  a  long,  narrow  lobe  than  across 
a  broad  one.  As  they  cross  the  lobe,  they  accelerate 
evaporation,  and  probably  lessen  cloudiness,  with  a  con- 


122  CLIMATIC  CHANGES 

sequent  augmentation  of  melting.  Moreover,  although 
lows  rarely  cross  a  broad  ice  sheet,  they  do  cross  a 
narrow  lobe.  For  example,  Nansen  records  that  strong 
lows  occasionally  cross  the  narrow  southern  part  of  the 
Greenland  ice  sheet.  The  longer  the  lobe,  the  more  likely 
it  is  that  lows  will  cross  it,  instead  of  following  its  mar- 
gin. Lows  which  cross  a  lobe  do  not  yield  so  much  snow 
to  the  tip  as  do  those  which  follow  the  margin.  Hence 
elongation  is  retarded  and  finally  stopped  even  without 
a  change  in  the  earth's  general  climate. 

Because  of  these  various  reasons  the  advances  of  the 
ice  during  the  several  epochs  of  a  glacial  period  might 
be  approximately  equal,  even  if  the  durations  of  the 
periods  of  storminess  and  low  temperature  were  differ- 
ent. Indeed,  they  might  be  sub-equal,  even  if  the  periods 
differed  in  intensity  as  well  as  length.  Differences  in  the 
periods  would  apparently  be  manifested  less  in  the  ex- 
tent of  the  ice  than  in  the  depth  of  glacial  erosion  and  in 
the  thickness  of  the  terminal  moraines,  outwash  plains, 
and  other  glacial  or  glacio-fluvial  formations. 

Having  completed  the  consideration  of  the  conditions 
leading  to  the  advance  of  the  ice,  let  us  now  consider  the 
condition  of  North  America  at  the  time  of  maximum 
glaciation.6  Over  an  area  of  nearly  four  million  square 
miles,  occupying  practically  all  the  northern  half  of  the 
continent  and  part  of  the  southern  half,  as  appears  in 
Fig.  6,  the  surface  was  a  monotonous  and  almost  level 
plain  of  ice  covered  with  snow.  When  viewed  from  a 
high  altitude,  all  parts  except  the  margins  must  have 
presented  a  uniformly  white  and  sparkling  appearance. 
Along  the  margins,  however,  except  to  the  north,  the 

«  The  description  of  the  distribution  of  the  ice  sheet  is  based  on  T.  C. 
Chamberlin's  wall  map  of  North  America  at  the  maximum  of  glaciation, 
1913. 


124  CLIMATIC  CHANGES 

whiteness  was  irregular,  for  the  view  must  have  included 
not  only  fresh  snow,  but  moving  clouds  and  dirty  snow 
or  ice.  Along  the  borders  where  melting  was  in  progress 
there  was  presumably  more  or  less  spottedness  due  to 
morainal  material  or  glacial  debris  brought  to  the  sur- 
face by  ice  shearage  and  wastage.  Along  the  dry  south- 
western border  it  is  also  possible  that  there  were  numer- 
ous dark  spots  due  to  dust  blown  onto  the  ice  by  the 
wind. 

The  great  white  sheet  with  its  ragged  border  was 
roughly  circular  in  form,  with  its  center  in  central 
Canada.  Yet  there  were  many  departures  from  a  per- 
fectly circular  form.  Some  were  due  to  the  oceans,  for, 
except  in  northern  Alaska,  the  ice  extended  into  the 
ocean  all  the  way  from  New  Jersey  around  by  the  north 
to  Washington.  On  the  south,  topographic  conditions 
made  the  margin  depart  from  a  simple  arc.  From  New 
Jersey  to  Ohio  it  swung  northward.  In  the  Mississippi 
Valley  it  reached  far  south;  indeed  most  of  the  broad 
wedge  between  the  Ohio  and  the  Missouri  rivers  was 
occupied  by  ice.  From  latitude  37°  near  the  junction  of 
the  Missouri  and  the  Mississippi,  however,  the  ice  margin 
extended  almost  due  north  along  the  Missouri  to  central 
North  Dakota.  It  then  stretched  westward  to  the  Rockies. 
Farther  west  lowland  glaciation  was  abundant  as  far 
south  as  western  Washington.  In  the  Rockies,  the  Cas- 
cades, and  the  Sierra  Nevadas  glaciation  was  common  as 
far  south  as  Colorado  and  southern  California,  respec- 
tively, and  snowfields  were  doubtless  extensive  enough  to 
make  these  ranges  ribbons  of  white.  Between  these  lofty 
ranges  lay  a  great  unglaciated  region,  but  even  in  the 
Great  Basin  itself,  in  spite  of  its  present  aridity,  certain 
ranges  carried  glaciers,  while  great  lakes  expanded 
widely. 


THE  GLACIAL  PERIOD  125 

In  this  vast  field  of  snow  the  glacial  ice  slowly  crept 
outward,  possibly  at  an  average  speed  of  half  a  foot  a 
day,  but  varying  from  almost  nothing  in  winter  at  the 
north,  to  several  feet  a  day  in  summer  at  the  south.7  The 
force  which  caused  the  movement  was  the  presence  of 
the  ice  piled  up  not  far  from  the  margins.  Almost  cer- 
tainly, however,  there  was  no  great  dome  from  the 
center  in  Canada  outward,  as  some  early  writers  as- 
sumed. Such  a  dome  would  require  that  the  ice  be  many 
thousands  of  feet  thick  near  its  center.  This  is  impos- 
sible because  of  the  fact  that  ice  is  more  voluminous  than 
water  (about  9  per  cent  near  the  freezing  point).  Hence 
when  subjected  to  sufficient  pressure  it  changes  to  the 
liquid  form.  As  friction  and  internal  heat  tend  to  keep 
the  bottom  of  a  glacier  warm,  even  in  cold  regions,  the 
probabilities  are  that  only  under  very  special  conditions 
was  a  continental  ice  sheet  much  thicker  than  about  2500 
feet.  In  Antarctica,  where  the  temperature  is  much  lower 
than  was  probably  attained  in  the  United  States,  the  ice 
sheet  is  nearly  level,  several  expeditions  having  traveled 
hundreds  of  miles  with  practically  no  change  in  altitude. 
In  Shackleton's  trip  almost  to  the  South  Pole,  he  en- 
countered a  general  rise  of  3000  feet  in  1200  miles.  Moun- 
tains, however,  projected  through  the  ice  even  near  the 
pole  and  the  geologists  conclude  that  the  ice  is  not  very 
thick  even  at  the  world 's  coldest  point,  the  South  Pole. 

Along  the  margin  of  the  ice  there  were  two  sorts  of 
movement,  much  more  rapid  than  the  slow  creep  of  the 
ice.  One  was  produced  by  the  outward  drift  of  snow 
carried  by  the  outblowing  dry  winds  and  the  other  and 
more  important  was  due  to  the  passage  of  cyclonic 
storms.  Along  the  border  of  the  ice  sheet,  except  at  the 

7  Chamberlin  and  Salisbury:  Geology,  1906,  Vol.  3,  and  W.  H.  Hobbs: 
Characteristics  of  Existing  Glaciers,  1911. 


126  CLIMATIC  CHANGES 

north,  storm  presumably  closely  followed  storm.  Their 
movement,  we  judge,  was  relatively  slow  until  near  the 
southern  end  of  the  Mississippi  lobe,  but  when  this  point 
was  passed  they  moved  much  more  rapidly,  for  then  they 
could  go  toward  instead  of  away  from  the  far  northern 
path  which  the  sun  prescribes  when  solar  activity  is 
great.  The  storms  brought  much  snow  to  the  icefield, 
perhaps  sometimes  in  favored  places  as  much  as  the  hun- 
dred feet  a  year  which  is  recorded  for  some  winters  in  the 
Sierras  at  present.  Even  the  unglaciated  intermontane 
Great  Basin  presumably  received  considerable  precipi- 
tation, perhaps  twice  as  much  as  its  present  scanty 
supply.  The  rainfall  was  enough  to  support  many  lakes, 
one  of  which  was  ten  times  as  large  as  Great  Salt  Lake ; 
and  grass  was  doubtless  abundant  upon  many  slopes 
which  are  now  dry  and  barren.  The  relatively  heavy 
precipitation  in  the  Great  Basin  was  probably  due  pri- 
marily to  the  increased  number  of  storms,  but  may  also 
have  been  much  influenced  by  their  slow  eastward  move- 
ment. The  lows  presumably  moved  slowly  in  that  general 
region  not  only  because  they  were  retarded  and  turned 
from  their  normal  path  by  the  cold  ice  to  the  east,  but 
because  during  the  summer  the  area  between  the  Sierra 
snowfields  on  the  west  and  the  Rocky  Mountain  and  Mis- 
sissippi Valley  snowfields  on  the  east  was  relatively 
warm.  Hence  it  was  normally  a  place  of  low  pressure 
and  therefore  of  inblowing  winds.  Slow-moving  lows  are 
much  more  effective  than  fast-moving  ones  in  drawing 
moisture  northwestward  from  the  Gulf  of  Mexico,  for 
they  give  the  moisture  more  time  to  move  spirally  first 
northeast,  under  the  influence  of  the  normal  south- 
westerly winds,  then  northwest  and  finally  southwest  as 
it  approaches  the  storm  center.  In  the  case  of  the  present 
lows,  before  much  moisture-laden  air  can  describe  such 


THE  GLACIAL  PERIOD  127 

a  circuit,  first  eastward  and  then  westward,  the  storm 
center  has  nearly  always  moved  eastward  across  the 
Kockies  and  even  across  the  Great  Plains.  A  result  of  this 
is  the  regular  decrease  in  precipitation  northward,  north- 
westward, and  westward  from  the  Gulf  of  Mexico. 

Along  the  part  of  the  glacial  margins  where  for  more 
than  3000  miles  the  North  American  ice  entered  the 
Atlantic  and  the  Pacific  oceans,  myriads  of  great  blocks 

broke  off  and  floated  away  as  stately  icebergs,  to  scatter   DJi  y „ 

boulders  far  over  the  ocean  floor  and  to  melt  in  warmer 
climes.  Where  the  margin  lay  upon  the  lands  numerous 
streams  issued  from  beneath  the  ice,  milk-white  with 
rock  flour,  and  built  up  great  outwash  plains  and  valley 
trains  of  gravel  and  sand.  Here  and  there,  just  beyond 
the  ice,  marginal  lakes  of  strange  shapes  occupied  valleys 
which  had  been  dammed  by  the  advancing  ice.  In  many 
of  them  the  water  level  rose  until  it  reached  some  low 
point  in  the  divide  and  then  overflowed,  forming  rapids 
and  waterfalls.  Indeed,  many  of  the  waterfalls  of  the 
eastern  United  States  and  Canada  were  formed  in  just 
this  way  and  not  a  few  streams  now  occupy  courses 
through  ridges  instead  of  parallel  to  them,  as  in  pre- 
glacial  times. 

In  the  zone  to  the  south  of  the  continental  ice  sheet, 
the  plant  and  animal  life  of  boreal,  cool  temperate,  and 
warm  temperate  regions  commingled  curiously.  Heather 
and  Arctic  willow  crowded  out  elm  and  oak;  musk  ox, 
hairy  mammoth,  and  marmot  contested  with  deer,  chip- 
munk, and  skunk  for  a  chance  to  live.  Near  the  ice  on 
slopes  exposed  to  the  cold  glacial  gales,  the  immigrant 
boreal  species  were  dominant,  but  not  far  away  in  more 
protected  areas  the  species  that  had  formerly  lived  there 
held  their  own.  In  Europe  during  the  last  two  advances 
of  the  great  ice  sheet  the  caveman  also  struggled  with 


128  CLIMATIC  CHANGES 

fierce  animals  and  a  fiercer  climate  to  maintain  life  in  an 
area  whose  habitability  had  long  been  decreasing. 

The  next  step  in  our  history  of  glaciation  is  to  outline 
the  disappearance  of  the  ice  sheets.  When  a  decrease  in 
solar  activity  produced  a  corresponding  decrease  in 
storminess,  several  influences  presumably  combined  to 
cause  the  disappearance  of  the  ice.  Most  of  their  results 
are  the  reverse  of  those  which  brought  on  glaciation.  A 
few  special  aspects,  however,  some  of  which  have  been 
discussed  in  Earth  and  Sun,  ought  to  be  brought  to  mind. 
A  diminution  in  storminess  lessens  upward  convection, 
wind  velocity,  and  evaporation,  and  these  changes,  if  they 
occurred,  must  have  united  to  raise  the  temperature  of 
the  lower  air  by  reducing  the  escape  of  heat.  Again  a 
decrease  in  the  number  and  intensity  of  tropical  cyclones 
presumably  lessened  the  amount  of  moisture  carried  into 
mid-latitudes,  and  thus  diminished  the  precipitation.  The 
diminution  of  snowfall  on  the  ice  sheets  when  storminess 
diminished  was  probably  highly  important.  The  amount 
of  precipitation  on  the  sheets  was  presumably  lessened 
still  further  by  changes  in  the  storminess  of  middle 
latitudes.  When  storminess  diminishes,  the  lows  follow  a 
less  definite  path,  as  Kullmer's  maps  show,  and  on  the 
average  a  more  southerly  path.  Thus,  instead  of  all  the 
lows  contributing  snow  to  the  ice  sheet,  a  large  fraction 
of  the  relatively  few  remaining  lows  would  bring  rain  to 
areas  south  of  the  ice  sheet.  As  storminess  decreased,  the 
trades  and  westerlies  probably  became  steadier,  and  thus 
carried  to  high  latitudes  more  warm  water  than  when 
often  interrupted  by  storms.  Steadier  southwesterly 
winds  must  have  produced  a  greater  movement  of  atmos- 
pheric as  well  as  oceanic  heat  to  high  latitudes.  The 
warming  due  to  these  two  causes  was  probably  the  chief 
reason  for  the  disappearance  of  the  European  ice  sheet 


THE  GLACIAL  PERIOD  129 

and  of  those  on  the  Pacific  coast  of  North  America.  The 
two  greater  American  ice  sheets,  however,  and  the 
glaciers  elsewhere  in  the  lee  of  high  mountain  ranges, 
probably  disappeared  chiefly  because  of  lessened  pre- 
cipitation. If  there  were  no  cyclonic  storms  to  draw  mois- 
ture northward  from  the  Gulf  of  Mexico,  most  of  North 
America  east  of  the  Rocky  Mountain  barrier  would  be 
arid.  Therefore  a  diminution  of  storminess  would  be 
particularly  effective  in  causing  the  disappearance  of  ice 
sheets  in  these  regions. 

That  evaporation  was  an  especially  important  factor 
in  causing  the  ice  from  the  Keewatin  center  to  disappear, 
is  suggested  by  the  relatively  small  amount  of  water- 
sorted  material  in  its  drift.  In  South  Dakota,  for  ex- 
ample, less  than  10  per  cent  of  the  drift  is  stratified.8  On 
the  other  hand,  Salisbury  estimates  that  perhaps  a  third 
of  the  Labradorean  drift  in  eastern  Wisconsin  is  crudely 
stratified,  about  half  of  that  in  New  Jersey,  and  more 
than  half  of  the  drift  in  western  Europe. 

When  the  sun's  activity  began  to  diminish,  all  these 
conditions,  as  well  as  several  others,  would  cooperate  to 
cause  the  ice  sheets  to  disappear.  Step  by  step  with  their 
disappearance,  the  amelioration  of  the  climate  would 
progress  so  long  as  the  period  of  solar  inactivity  con- 
tinued and  storms  were  rare.  If  the  inactivity  continued 
long  enough,  it  would  result  in  a  fairly  mild  climate  in 
high  latitudes,  though  so  long  as  the  continents  were 
emergent  this  mildness  would  not  be  of  the  extreme  type. 
The  inauguration  of  another  cycle  of  increased  disturb- 
ance of  the  sun,  with  a  marked  increase  in  storminess, 
would  inaugurate  another  glacial  epoch.  Thus  a  succes- 
sion of  glacial  and  inter-glacial  epochs  might  continue  so 
long  as  the  sun  was  repeatedly  disturbed. 

s  S.  S.  Visher:  The  Geography  of  South  Dakota;  S.  D.  Geol.  Surv.,  1918. 


CHAPTER  VIII 
SOME  PROBLEMS  OF  GLACIAL  PERIODS 

HAVING  outlined  in  general  terms  the  coming  of 
the  ice  sheets  and  their  disappearance,  we  are 
now  ready  to  discuss  certain  problems  of  com- 
pelling climatic  interest.  The  discussion  will  be  grouped 
under  five  heads:  (I)  the  localization  of  glaciation;  (II) 
the  sudden  coming  of  glaciation;  (III)  peculiar  varia- 
tions in  the  height  of  the  snow  line  and  of  glaciation; 
(IV)  lakes  and  other  evidences  of  humidity  in  ungla- 
ciated  regions  during  the  glacial  epochs ;  ( V)  glaciation 
at  sea  level  and  in  low  latitudes  in  the  Permian  and 
Proterozoic  eras.  The  discussion  of  perhaps  the  most 
difficult  of  all  climatic  problems  of  glaciation,  that  of  the 
succession  of  cold  glacial  and  mild  inter-glacial  epochs, 
has  been  postponed  to  the  next  to  the  final  chapter  of  this 
book.  It  cannot  be  properly  considered  until  we  take  up 
the  history  of  solar  disturbances. 

I.  The  first  problem,  the  localization  of  the  ice  sheets, 
arises  from  the  fact  that  in  both  the  Pleistocene  and  the 
Permian  periods  glaciation  was  remarkably  limited.  In 
neither  period  were  all  parts  of  high  latitudes  glaciated ; 
yet  in  both  cases  glaciation  occurred  in  large  regions  in 
lower  latitudes.  Many  explanations  of  this  localization 
have  been  offered,  but  most  are  entirely  inadequate.  Even 
hypotheses  with  something  of  proven  worth,  such  as 
those  of  variations  in  volcanic  dust  and  in  atmospheric 


SOME  PROBLEMS  OF  GLACIAL  PERIODS      131 

carbon  dioxide,  fail  to  account  for  localization.  The 
cyclonic  form  of  the  solar  hypothesis,  however,  seems  to 
afford  a  satisfactory  explanation. 

The  distribution  of  the  ice  in  the  last  glacial  period  is 
well  known,  and  is  shown  in  Fig.  6.  Four-fifths  of  the 
ice-covered  area,  which  was  eight  million  square  miles, 
more  or  less,  was  near  the  borders  of  the  North  Atlantic 
in  eastern  North  America  and  northwestern  Europe. 
The  ice  spread  out  from  two  great  centers  in  North 
America,  the  Labradorean  east  of  Hudson  Bay,  and  the 
Keewatin  west  of  the  bay.  There  were  also  many  glaciers 
in  the  western  mountains,  especially  in  Canada,  while 
subordinate  centers  occurred  in  Newfoundland,  the  Adi- 
rondacks,  and  the  White  Mountains.  The  main  ice  sheet 
at  its  maximum  extension  reached  as  far  south  as  lati- 
tude 39°  in  Kansas  and  Kentucky,  and  37°  in  Illinois. 
Huge  boulders  were  transferred  more  than  one  thousand 
miles  from  their  source  in  Canada.  The  northward  ex- 
tension was  somewhat  less.  Indeed,  the  northern  margin 
of  the  continent  was  apparently  relatively  little  glaciated 
and  much  of  Alaska  unglaciated.  Why  should  northern 
Kentucky  be  glaciated  when  northern  Alaska  was  not? 

In  Europe  the  chief  center  from  which  the  continental 
glacier  moved  was  the  Scandinavian  highlands.  It  pushed 
across  the  depression  now  occupied  by  the  Baltic  to 
southern  Russia  and  across  the  North  Sea  depression 
to  England  and  Belgium.  The  Alps  formed  a  center  of 
considerable  importance,  and  there  were  minor  centers 
in  Scotland,  Ireland,  the  Pyrenees,  Apennines,  Caucasus, 
and  Urals.  In  Asia  numerous  ranges  also  contained  large 
glaciers,  but  practically  all  the  glaciation  was  of  the 
alpine  type  and  very  little  of  the  vast  northern  lowland 
was  covered  with  ice. 

In  the  southern  hemisphere  glaciation  at  low  latitudes 


132  CLIMATIC  CHANGES 

was  less  striking  than  in  the  northern  hemisphere.  Most 
of  the  increase  in  the  areas  of  ice  was  confined  to  moun- 
tains which  today  receive  heavy  precipitation  and  still 
contain  small  glaciers.  Indeed,  except  for  relatively  slight 
glaciation  in  the  Australian  Alps  and  in  Tasmania,  most 
of  the  Pleistocene  glaciation  in  the  southern  hemisphere 
was  merely  an  extension  of  existing  glaciers,  such  as 
those  of  south  Chile,  New  Zealand,  and  the  Andes.  Never- 
theless, fairly  extensive  glaciation  existed  much  nearer 
the  equator  than  is  now  the  case. 

In  considering  the  localization  of  Pleistocene  glacia- 
tion, three  main  factors  must  be  taken  into  account, 
namely,  temperature,  topography,  and  precipitation.  The 
absence  of  glaciation  in  large  parts  of  the  Arctic  regions 
of  North  America  and  of  Asia  makes  it  certain  that  low 
temperature  was  not  the  controlling  factor.  Aside  from 
Antarctica,  the  coldest  place  in  the  world  is  northeastern 
Siberia.  There  for  seven  months  the  average  temperature 
is  below  0°C.,  while  the  mean  for  the  whole  year  is 
below  — 10°  C.  If  the  temperature  during  a  glacial  period 
averaged  6°C.  lower  than  now,  as  is  commonly  supposed, 
this  part  of  Siberia  would  have  had  a  temperature  below 
freezing  for  at  least  nine  months  out  of  the  twelve  even  if 
there  were  no  snowfield  to  keep  the  summers  cold.  Yet 
even  under  such  conditions  no  glaciation  occurred,  al- 
though in  other  places,  such  as  parts  of  Canada  and 
northwestern  Europe,  intense  glaciation  occurred  where 
the  mean  temperature  is  much  higher. 

The  topography  of  the  lands  apparently  had  much 
more  influence  upon  the  localization  of  glaciation  than 
did  temperature.  Its  effect,  however,  was  always  to  cause 
glaciation  exactly  where  it  would  be  expected  and  not  in 
unexpected  places  as  actually  occurred.  For  example,  in 
North  America  the  western  side  of  the  Canadian  Rockies 


SOME  PROBLEMS  OF  GLACIAL  PERIODS      133 

suffered  intense  glaciation,  for  there  precipitation  was 
heavy  because  the  westerly  winds  from  the  Pacific  are 
forced  to  give  up  their  moisture  as  they  rise.  In  the  same 
way  the  western  side  of  the  Sierra  Nevadas  was  much 
more  heavily  glaciated  than  the  eastern  side.  In  similar 
fashion  the  windward  slopes  of  the  Alps,  the  Caucasus, 
the  Himalayas,  and  many  other  mountain  ranges  suf- 
fered extensive  glaciation.  Low  temperature  does  not  ^  -4~cJ*> 
seem  to  have  been  the  cause  of  this  glaciation,  for  in  that 
case  it  is  hard  to  see  why  both  sides  of  the  various  ranges  0~i 
did  not  show  an  equal  percentage  of  increase  in  the  size 
of  their  icefields. 

From  what  has  been  said  as  to  temperature  and  topog- 
raphy, it  is  evident  that  variations  in  precipitation  have 
had  much  more  to  do  with  glaciation  than  have  variations 
in  temperature.  In  the  Arctic  lowlands  and  on  the  lee- 
ward side  of  mountains,  the  slight  development  of  glacia- 
tion appears  to  have  been  due  to  scarcity  of  precipita- 
tion. On  the  windward  side  of  mountains,  on  the  other 
hand,  a  notable  increase  in  precipitation  seems  to  have 
led  to  abundant  glaciation.  Such  an  increase  in  precipi- 
tation must  be  dependent  on  increased  evaporation  and 
this  could  arise  either  from  relatively  high  temperature 
or  strong  winds.  Since  the  temperature  in  the  glacial 
period  was  lower  than  now,  we  seem  forced  to  attributep^Jy^  r 
the  increased  precipitation  to  a  strengthening  of  the' 
winds.  If  the  westerly  winds  from  the  Pacific  should  in- 
crease in  strength  and  waft  more  moisture  to  the  western 
side  of  the  Canadian  Eockies,  or  if  similar  winds  in- 
creased the  snowfall  on  the  upper  slopes  of  the  Alps  or 
the  Tian-Shan  Mountains,  the  glaciers  would  extend 
lower  than  now  without  any  change  in  temperature. 

Although  the  incompetence  of  low  temperature  to  cause 
glaciation,  and  the  relative  unimportance  of  the  moun- 


134  CLIMATIC  CHANGES 

tains  in  northeastern  Canada  and  northwestern  Europe 
throw  most  glacial  hypotheses  out  of  court,  they  are  in 
harmony  with  the  cyclonic  hypothesis.  The  answer  of 
that  hypothesis  to  the  problem  of  the  localization  of  ice 
sheets  seems  to  be  found  in  certain  maps  of  storminess 
and  rainfall  in  relation  to  solar  activity.  In  Fig.  2  a 
marked  belt  of  increased  storminess  at  times  of  many 
sunspots  is  seen  in  southern  Canada.  A  comparison  of 
this  with  a  series  of  maps  given  in  Earth  and  Sun  shows 
that  the  stormy  belt  tends  to  migrate  northward  in  har- 
mony with  an  increase  in  the  activity  of  the  sun's  atmos- 
phere. If  the  sun  were  sufficiently  active  the  belt  of 
maximum  storminess  would  apparently  pass  through  the 
Keewatin  and  Labradorean  centers  of  glaciation  instead 
of  well  to  the  south  of  them,  as  at  present.  It  would 
presumably  cross  another  center  in  Greenland,  and  then 
would  traverse  the  fourth  of  the  great  centers  of  Pleisto- 
cene glaciation  in  Scandinavia.  It  would  not  succeed  in 
traversing  northern  Asia,  however,  any  more  than  it 
does  now,  because  of  the  great  high-pressure  area  which 
develops  there  in  winter.  When  the  ice  sheets  expanded 
from  the  main  centers  of  glaciation,  the  belt  of  storms 
would  be  pushed  southward  and  outward.  Thus  it  might 
give  rise  to  minor  centers  of  glaciers  such  as  the  Patri- 
cian between  Hudson  Bay  and  Lake  Superior,  or  the 
centers  in  Ireland,  Cornwall,  Wales,  and  the  northern 
Ural  Mountains.  As  the  main  ice  sheets  advanced,  how- 
ever, the  minor  centers  would  be  overridden  and  the 
entire  mass  of  ice  would  be  merged  into  one  vast  expanse 
in  the  Atlantic  portion  of  each  of  the  two  continents. 

In  this  connection  it  may  be  well  to  consider  briefly  the 
most  recent  hypothesis  as  to  the  growth  and  hence  the 
localization  of  glaciation.  In  1911  and  more  fully  in  1915, 


SOME  PROBLEMS  OF  GLACIAL  PERIODS      135 

Hobbs,1  advanced  the  anti-cyclonic  hypothesis  of  the 
origin  of  ice  sheets.  This  hypothesis  has  the  great  merit 
of  focusing  attention  upon  the  fact  that  ice  sheets  are 
pronounced  anti-cyclonic  regions  of  high  pressure.  This 
is  proved  by  the  strong  outblowing  winds  which  pre- 
vail along  their  margins.  Such  winds  must,  of  course,  be 
balanced  by  inward-moving  winds  at  high  levels.  Abun- 
dant observations  prove  that  such  is  the  case.  For 
example,  balloons  sent  up  by  Barkow  near  the  margin  of 
the  Antarctic  ice  sheet  reveal  the  occurrence  of  inblow- 
ing  winds,  although  they  rarely  occur  below  a  height  of 
9000  meters.  The  abundant  data  gathered  by  Guervain 
on  the  coast  of  Greenland  indicate  that  outblowing  winds 
prevail  up  to  a  height  of  about  4000  meters.  At  that 
height  inblowing  winds  commence  and  increase  in  fre- 
quency until  at  an  altitude  of  over  5000  meters  they  be- 
come more  common  than  outblowing  winds.  It  should  be 
noted,  however,  that  in  both  Antarctica  and  Greenland, 
although  the  winds  at  an  elevation  of  less  than  a  thousand 
meters  generally  blow  outward,  there  are  frequent  and 
decided  departures  from  this  rule,  so  that  "variable 
winds"  are  quite  commonly  mentioned  in  the  reports  of 
expeditions  and  balloon  soundings. 

The  undoubted  anti-cyclonic  conditions  which  Hobbs 
thus  calls  to  the  attention  of  scientists  seem  to  him  to 
necessitate  a  peculiar  mechanism  in  order  to  produce 
the  snow  which  feeds  the  glaciers.  He  assumes  that  the 
winds  which  blow  toward  the  centers  of  the  ice  sheets 
at  high  levels  carry  the  necessary  moisture  by  which  the 
glaciers  grow.  When  the  air  descends  in  the  centers  of 
the  highs,  it  is  supposed  to  be  chilled  on  reaching  the  sur- 

iW.  H.  Hobbs:  Characteristics  of  Existing  Glaciers,  1911.  The  Eole  of 
the  Glacial  Anticyclones  in  the  Air  Circulation  of  the  Globe;  Proc.  Am. 
Phil.  Soc.,  Vol.  54,  1915,  pp.  185-225. 


136  CLIMATIC  CHANGES 

face  of  the  ice,  and  hence  to  give  up  its  moisture  in  the 
form  of  minute  crystals.  This  conclusion  is  doubtful  for 
several  reasons.  In  the  first  place,  Hobbs  does  not  seem 
to  appreciate  the  importance  of  the  variable  winds  which 
he  quotes  Arctic  and  Antarctic  explorers  as  describing 
quite  frequently  on  the  edges  of  the  ice  sheets.  They  are 
one  of  many  signs  that  cyclonic  storms  are  fairly  fre- 
quent on  the  borders  of  the  ice  though  not  in  its  interior. 
Thus  there  is  a  distinct  and  sufficient  form  of  precipita- 
tion actually  at  work  near  the  margin  of  the  ice,  or 
exactly  where  the  thickness  of  the  ice  sheet  would  lead 
us  to  expect. 

Another  consideration  which  throws  grave  doubt  on 
the  anti-cyclonic  hypothesis  of  ice  sheets  is  the  small 
amount  of  moisture  possible  in  the  highs  because  of  their 
low  temperature.  Suppose,  for  the  sake  of  argument, 
that  the  temperature  in  the  middle  of  an  ice  sheet  aver- 
ages 20°F.  This  is  probably  much  higher  than  the  actual 
fact  and  therefore  unduly  favorable  to  the  anti-cyclonic 
hypothesis.  Suppose  also  that  the  decrease  in  tempera- 
ture from  the  earth's  surface  upward  proceeds  at  the 
rate  of  1°F.  for  each  300  feet,  which  is  50  per  cent  less 
than  the  actual  rate  for  air  with  only  a  slight  amount  of 
moisture,  such  as  is  found  in  cold  regions.  Then  at  a 
height  of  10,000  feet,  where  the  inblowing  winds  begin 
to  be  felt,  the  temperature  would  be  — 20°F.  At  that 
temperature  the  air  is  able  to  hold  approximately  0.166 
grain  of  moisture  per  cubic  foot  when  fully  saturated. 
This  is  an  exceedingly  small  amount  of  moisture  and  even 
if  it  were  all  precipitated  could  scarcely  build  a  glacier. 
However,  it  apparently  would  not  be  precipitated  because 
when  such  air  descends  in  the  center  of  the  anti-cyclone 
it  is  warmed  adiabatically,  that  is,  by  compression.  On 
reaching  the  surface  it  would  have  a  temperature  of  20° 


SOME  PROBLEMS  OF  GLACIAL  PERIODS      137 

and  would  be  able  to  hold  0.898  grain  of  water  vapor  per 
cubic  foot;  in  other  words,  it  would  have  a  relative  hu- 
midity of  about  18  per  cent.  Under  no  reasonable  assump- 
tion does  the  upper  air  at  the  center  of  an  ice  sheet 
appear  to  reach  the  surface  with  a  relative  humidity  of 
more  than  20  or  25  per  cent.  Such  air  cannot  give  up 
moisture.  On  the  contrary,  it  absorbs  it  and  tends  to 
diminish  rather  than  increase  the  thickness  of  the  sheet 
of  ice  and  snow.  But  after  the  surplus  heat  gained  by 
descent  has  been  lost  by  radiation,  conduction,  and 
evaporation,  the  air  may  become  super-saturated  with 
the  moisture  picked  up  while  warm.  Hobbs  reports  that 
explorers  in  Antarctica  and  Greenland  have  frequently 
observed  condensation  on  their  clothing.  If  such  moisture 
is  not  derived  directly  from  the  men's  own  bodies,  it  is 
apparently  picked  up  from  the  ice  sheet  by  the  descending 
air,  and  not  added  to  the  ice  sheet  by  air  from  aloft. 

The  relation  of  all  this  to  the  localization  of  ice  sheets 
is  this.  If  Hobbs'  anti-cyclonic  hypothesis  of  glacial 
growth  is  correct,  it  would  appear  that  ice  sheets  should 
grow  up  where  the  temperature  is  lowest  and  the  high- 
pressure  areas  most  persistent ;  for  instance,  in  northern 
Siberia.  It  would  also  appear  that  so  far  as  the  topog- 
raphy permitted,  the  ice  sheets  ought  to  move  out  uni- 
formly in  all  directions ;  hence  the  ice  sheet  ought  to  be 
as  prominent  to  the  north  of  the  Keewatin  and  Labra- 
dorean  centers  as  to  the  south,  which  is  by  no  means  the 
case.  Again,  in  mountainous  regions,  such  as  the  glacial 
areas  of  Alaska  and  Chile,  the  glaciation  ought  not  to 
be  confined  to  the  windward  slope  of  the  mountains  so 
closely  as  is  actually  the  fact.  In  each  of  these  cases  the 
glaciated  region  was  large  enough  so  that  there  was 
probably  a  true  anti-cyclonic  area  comparable  with  that 
now  prevailing  over  southern  Greenland.  In  both  places 


138  CLIMATIC  CHANGES 

the  correlation  between  glaciation  and  mountain  ranges 
seems  much  too  close  to  support  the  anti-cyclonic  hy- 
pothesis, for  the  inblowing  winds  which  on  that  hypothe- 
sis bring  the  moisture  are  shown  by  observation  to  occur 
at  heights  far  greater  than  that  of  all  but  the  loftiest 
ranges. 

II.  The  sudden  coming  of  glaciation  is  another  prob- 
lem which  has  been  a  stumbling-block  in  the  way  of  every 
glacial  hypothesis.  In  his  Climates  of  Geologic  Times, 
Schuchert  states  that  the  fossils  give  almost  no  warning 
of  an  approaching  catastrophe.  If  glaciation  were  solely 
due  to  uplift,  or  other  terrestrial  changes  aside  from  vul- 
canism,  Schuchert  holds  that  it  would  have  come  slowly 
and  the  stages  preceding  glaciation  would  have  affected 
life  sufficiently  to  be  recorded  in  the  rocks.  He  considers 
that  the  suddenness  of  the  coming  of  glaciation  is  one 
of  the  strongest  arguments  against  the  carbon  dioxide 
hypothesis  of  glaciation. 

According  to  the  cyclonic  hypothesis,  however,  the 
suddenness  of  the  oncoming  of  glaciation  is  merely  what 
would  be  expected  on  the  basis  of  what  happens  today. 
Changes  in  the  sun  occur  suddenly.  The  sunspot  cycle  is 
only  eleven  or  twelve  years  long,  and  even  this  short 
period  of  activity  is  inaugurated  more  suddenly  than  it 
declines.  Again  the  climatic  record  derived  from  the 
growth  of  trees,  as  given  in  Figs.  4  and  5,  also  shows  that 
marked  changes  in  climate  are  initiated  more  rapidly 
than  they  disappear.  In  this  connection,  however,  it  must 
be  remembered  that  solar  activity  may  arise  in  various 
ways,  as  will  appear  more  fully  later.  Under  certain  con- 
ditions storminess  may  increase  and  decrease  slowly. 

III.  The  height  of  the  snow  line  and  of  glaciation  fur- 
nishes another  means  of  testing  glacial  hypotheses.  It  is 
well  established  that  in  times  of  glaciation  the  snow  line 


SOME  PROBLEMS  OF  GLACIAL  PERIODS      139 

was  depressed  everywhere,  but  least  near  the  equator. 
For  example,  according  to  Penck,  permanent  snow  ex- 
tended 4000  feet  lower  than  now  in  the  Alps,  whereas 
it  stood  only  1500  feet  below  the  present  level  near  the 
equator  in  Venezuela.  This  unequal  depression  is  not 
readily  accounted  for  by  any  hypothesis  depending  solely 
upon  the  lowering  of  temperature.  By  the  carbon  dioxide 
and  the  volcanic  dust  hypotheses,  the  temperature  pre- 
sumably was  lowered  amost  equally  in  all  latitudes,  but 
a  little  more  at  the  equator  than  elsewhere.  If  glaciation 
were  due  to  a  temporary  lessening  of  the  radiation  re- 
ceived from  the  sun,  such  as  is  demanded  by  the  thermal 
solar  hypothesis,  and  by  the  longer  periods  of  Croll's 
hypothesis,  the  lowering  would  be  distinctly  greatest  at 
the  equator.  Thus,  according  to  all  these  hypotheses,  the 
snow  line  should  have  been  depressed  most  at  the  equator, 
instead  of  least. 

The  cyclonic  hypothesis  explains  the  lesser  depression 
of  the  snow  line  at  the  equator  as  due  to  a  diminution  of 
precipitation.  The  effectiveness  of  precipitation  in  this 
respect  is  illustrated  by  the  present  great  difference  in 
the  height  of  the  snow  line  on  the  humid  and  dry  sides  of 
mountains.  On  the  wet  eastern  side  of  the  Andes  near  the 
equator,  the  snow  line  lies  at  16,000  feet;  on  the  dry 
western  side,  at  18,500  feet.  Again,  although  the  humid 
side  of  the  Himalayas  lies  toward  the  south,  the  snow  line 
has  a  level  of  15,000  feet,  while  farther  north,  on  the  dry 
side,  it  is  16,700  feet.2  The  fact  that  the  snow  line  is  lower 
near  the  margin  of  the  Alps  than  toward  the  center 
points  in  the  same  direction.  The  bearing  of  all  this  on 
the  glacial  period  may  be  judged  by  looking  again  at  Fig. 
3  in  Chapter  V.  This  shows  that  at  times  of  sunspot 
activity  and  hence  of  augmented  storminess,  the  precipi- 

2E.  D.  Salisbury:  Physiography,  1919. 


140  CLIMATIC  CHANGES 

tation  diminishes  near  the  heat  equator,  that  is,  where 
the  average  temperature  for  the  whole  year  is  highest. 
At  present  the  great  size  of  the  northern  continents  and 
their  consequent  high  temperature  in  summer,  cause  the 
heat  equator  to  lie  north  of  the  "real"  equator,  except 
where  Australia  draws  it  to  the  southward.3  When  large 
parts  of  the  northern  continents  were  covered  with  ice, 
however,  the  heat  equator  and  the  true  equator  were 
probably  much  closer  than  now,  for  the  continents  could 
not  become  so  hot.  If  so,  the  diminution  in  equatorial 
precipitation,  which  accompanies  increased  storminess 
throughout  the  world  as  a  whole,  would  take  place  more 
nearly  along  the  true  equator  than  appears  in  Fig.  3. 
Hence  so  far  as  precipitation  alone  is  concerned,  we 
should  actually  expect  that  the  snow  line  near  the  equator 
would  rise  a  little  during  glacial  periods.  Another  factor, 
however,  must  be  considered.  Koppen's  data,  it  will  be 
remembered,  show  that  at  times  of  solar  activity  the 
earth's  temperature  falls  more  at  the  equator  than  in 
higher  latitudes.  If  this  effect  were  magnified  it  would 
lower  the  snow  line.  The  actual  position  of  the  snow  line 
at  the  equator  during  glacial  periods  thus  appears  to  be 
the  combined  effect  of  diminished  precipitation,  which 
would  raise  the  line,  and  of  lower  temperature,  which 
would  bring  it  down. 

Before  leaving  this  subject  it  may  be  well  to  recall  that 
the  relative  lessening  of  precipitation  in  equatorial  lati- 
tudes during  the  glacial  epochs  was  probably  caused  by 
the  diversion  of  moisture  from  the  trade-wind  belt.  This 
diversion  was  presumably  due  to  the  great  number  of 
tropical  cyclones  and  to  the  fact  that  the  cyclonic  storms 
of  middle  latitudes  also  drew  much  moisture  from  the 
trade-wind  belt  in  summer  when  the  northern  position  of 

s  Griffith  Taylor:  Australian  Meteorology,  1920,  p.  283. 


SOME  PROBLEMS  OF  GLACIAL  PERIODS      141 

the  sun  drew  that  belt  near  the  storm  track  which  was 
forced  to  remain  south  of  the  ice  sheet.  Such  diversion 
of  moisture  out  of  the  trade-wind  belt  must  diminish  the 
amount  of  water  vapor  that  is  carried  by  the  trades  to 
equatorial  regions;  hence  it  would  lessen  precipitation 
in  the  belt  of  so-called  equatorial  calms,  which  lies  along 
the  heat  equator  rather  than  along  the  geographical 
equator. 

Another  phase  of  the  vertical  distribution  of  glaciation 
has  been  the  subject  of  considerable  discussion.  In  the 
Alps  and  in  many  other  mountains  the  glaciation  of  the 
Pleistocene  period  appears  to  have  had  its  upper  limit 
no  higher  than  today.  This  has  been  variously  inter- 
preted. It  seems,  however,  to  be  adequately  explained 
as  due  to  decreased  precipitation  at  high  altitudes  during 
the  cold  periods.  This  is  in  spite  of  the  fact  that  precipi- 
tation in  general  increased  with  increased  storminess. 
The  low  temperature  of  glacial  times  presumably  induced 
condensation  at  lower  altitudes  than  now,  and  most  of 
the  precipitation  occurred  upon  the  lower  slopes  of  the 
mountains,  contributing  to  the  lower  glaciers,  while  little 
of  it  fell  upon  the  highest  glaciers.  Above  a  moderate 
altitude  in  all  lofty  mountains  the  decrease  in  the  amount 
of  precipitation  is  rapid.  In  most  cases  the  decrease 
begins  at  a  height  of  less  than  3000  feet  above  the  base 
of  the  main  slope,  provided  the  slope  is  steep.  The  colder 
the  air,  the  lower  the  altitude  at  which  this  occurs.  For 
example,  it  is  much  lower  in  winter  than  in  summer. 
Indeed,  the  higher  altitudes  in  the  Alps  are  sunny  in 
winter  even  where  there  are  abundant  clouds  lower  down. 

IV.  The  presence  of  extensive  lakes  and  other  evidences 
of  a  pluvial  climate  during  glacial  periods  in  non-glaci- 
ated regions  which  are  normally  dry  is  another  of  the 
facts  which  most  glacial  hypotheses  fail  to  explain  satis- 


142  CLIMATIC  CHANGES 

factorily.  Beyond  the  ice  sheets  many  regions  appear  to 
have  enjoyed  an  unusually  heavy  precipitation  during  the 
glacial  epochs.  The  evidence  of  this  is  abundant,  includ- 
ing numerous  abandoned  strand  lines  of  salt  lakes  and  an 
abundance  of  coarse  material  in  deltas  and  flood  plains. 
J.  D.  Whitney,4  in  an  interesting  but  neglected  volume, 
was  one  of  the  first  to  marshal  the  evidence  of  this  sort. 
More  recently  Free5  has  amplified  this.  According  to  him 
in  the  Great  Basin  region  of  the  United  States  sixty-two 
basins  either  contain  unmistakable  evidence  of  lakes,  or 
belong  to  one  of  the  three  great  lake  groups  named  below. 
Two  of  these,  the  Lake  Lahontan  and  the  Lake  Bonne- 
ville  groups,  comprise  twenty-nine  present  basins,  while 
the  third,  the  Owens-Searles  chain,  contained  at  least 
five  large  lakes,  the  lowest  being  in  Death  Valley.  In 
western  and  central  Asia  a  far  greater  series  of  salt  lakes 
is  found  and  most  of  these  are  surrounded  by  strands  at 
high  levels.  Many  of  these  are  described  in  Explorations 
in  Turkestan,  The  Pulse  of  Asia,  and  Palestine  and  Its 
Transformation.  There  has  been  a  good  deal  of  debate  as 
to  whether  these  lakes  actually  date  from  the  glacial 
period,  as  is  claimed  by  C.  E.  P.  Brooks,  for  example,  or 
from  some  other  period.  The  evidence,  however,  seems  to 
be  convincing  that  the  lakes  expanded  when  the  ice  also 
expanded. 

According  to  the  older  glacial  hypotheses  the  lower 
temperature  which  is  postulated  as  the  cause  of  glacia- 
tion  would  almost  certainly  mean  less  evaporation  over 
the  oceans  and  hence  less  precipitation  during  glacial 
periods.  To  counteract  this  the  only  way  in  which  the 

*  J.  D.  Whitney:  Climatic  Changes  of  the  Later  Geological  Times,  1882. 

5E.  E.  Free:  U.  S.  Dept.  of  Agriculture,  Bull.  54,  1914.  Mr.  Free  has 
prepared  a  summary  of  this  Bulletin  which  appears  in  The  Solar  Hypothesis, 
Bull.  Geol.  Soe.  of  Am.,  Vol.  25,  pp.  559-562. 


SOME  PROBLEMS  OF  GLACIAL  PERIODS      143 

level  of  the  lakes  could  be  raised  would  be  because  the 
lower  temperature  would  cause  less  evaporation  from 
their  surfaces.  It  seems  quite  impossible,  however,  that 
the  lowering  of  temperature,  which  is  commonly  taken 
to  have  been  not  more  than  10°  C.,  could  counteract  the 
lessened  precipitation  and  also  cause  an  enormous  ex- 
pansion of  most  of  the  lakes.  For  example,  ancient  Lake 
Bonneville  was  more  than  ten  times  as  large  as  its 
modern  remnant,  Great  Salt  Lake,  and  its  average  depth 
more  than  forty  times  as  great.6  Many  small  lakes  in  the 
Old  World  expanded  still  more.7  For  example,  in  eastern 
Persia  many  basins  which  now  contain  no  lake  whatever 
are  floored  with  vast  deposits  of  lacustrine  salt  and  are 
surrounded  by  old  lake  bluffs  and  beaches.  In  northern 
Africa  similar  conditions  prevail.8  Other,  but  less  ob- 
vious, evidence  of  more  abundant  rainfall  in  regions  that 
are  now  dry  is  found  in  thick  strata  of  gravel,  sand,  and 
fine  silt  in  the  alluvial  deposits  of  flood  plains  and  deltas.9 
The  cyclonic  hypothesis  supposes  that  increased 
storminess  accounts  for  pluvial  climates  in  regions  that 
are  now  dry  just  as  it  accounts  for  glaciation  in  the 
regions  of  the  ice  sheets.  Figs.  2  and  3,  it  will  be  remem- 
bered, illustrate  what  happens  when  the  sun  is  active. 
Solar  activity  is  accompanied  by  an  increase  in  stormi- 
ness in  the  southwestern  United  States  in  exactly  the  re- 
gion where  elevated  strands  of  diminished  salt  lakes  are 
most  numerous.  In  Fig.  3,  the  same  condition  is  seen  in 

«G.  K.  Gilbert:  Lake  Bonneville;  Monograph  1,  U.  S.  Geol.  Surv. 

7  C.  E.  P.  Brooks:  Quart.  Jour.  Royal  Meteorol.  Soe.,  1914,  pp.  63-66. 

s  H.  J.  L.  Beadnell :  An  Egyptian  Oasis,  London,  1909. 

Ellsworth  Huntington:  The  Libyan  Oasis  of  Kharga;  Bull.  Am.  Geog. 
Soc.,  Vol.  42,  Sept.,  1910,  pp.  641-661. 

»S.  S.  Visher:  The  Bajada  of  the  Tucson  Bolson  of  Southern  Arizona; 
Science,  N.  S.,  Mar.  23,  1913. 

Ellsworth  Huntington:  The  Basins  of  Eastern  Persia  and  Seistan,  in 
Explorations  in  Turkestan. 


144  CLIMATIC  CHANGES 

the  region  of  salt  lakes  in  the  Old  World.  Judging  by 
these  maps,  which  illustrate  what  has  happened  since 
careful  meteorological  records  were  kept,  an  increase  in 
solar  activity  is  accompanied  by  increased  rainfall  in 
large  parts  of  what  are  now  semi-arid  and  desert  regions. 
Such  precipitation  would  at  once  cause  the  level  of  the 
lakes  to  rise.  Later,  when  ice  sheets  had  developed  in 
Europe  and  America,  the  high-pressure  areas  thus  caused 
might  force  the  main  storm  belt  so  far  south  that  it  would 
lie  over  these  same  arid  regions.  The  increase  in  tropical 
hurricanes  at  times  of  abundant  sunspots  may  also  have 
a  bearing  on  the  climate  of  regions  that  are  now  arid. 
During  the  glacial  period  some  of  the  hurricanes  prob- 
ably swept  far  over  the  lands.  The  numerous  tropical 
cyclones  of  Australia,  for  example,  are  the  chief  source 
T  of  precipitation  for  that  continent.10  Some  of  the  stronger 
cyclones  locally  yield  more  rain  in  a  day  or  two  than 
other  sources  yield  in  a  year. 

V.  The  occurrence  of  widespread  glaciation  near  the 
tropics  during  the  Permian,  as  shown  in  Fig.  7,  has  given 
rise  to  much  discussion.  The  recent  discovery  of  glacia- 
tion in  latitudes  as  low  as  30°  in  the  Proterozoic  is  corre- 
spondingly significant.  In  all  cases  the  occurrence  of 
glaciation  in  low  and  middle  latitudes  is  probably  due  to 
the  same  general  causes.  Doubtless  the  position  and  alti- 
tude of  the  mountains  had  something  to  do  with  the 
matter.  Yet  taken  by  itself  this  seems  insufficient.  Today 
the  loftiest  range  in  the  world,  the  Himalayas,  is  almost 
unglaciated,  although  its  southern  slope  may  seem  at  first 
thought  to  be  almost  ideally  located  in  this  respect.  Some 
parts  rise  over  20,000  feet  and  certain  lower  slopes  re- 
ceive 400  inches  of  rain  per  year.  The  small  size  of  the 
Himalayan  glaciers  in  spite  of  these  favorable  conditions 

10  Griffith  Taylor:  Australian  Meteorology,  1920,  p.  189. 


1 

f! 


146  CLIMATIC  CHANGES 

is  apparently  due  largely  to  the  seasonal  character  of  the 
monsoon  winds.  The  strong  outblowing  monsoons  of 
winter  cause  about  half  the  year  to  be  very  dry  with  clear 
skies  and  dry  winds  from  the  interior  of  Asia.  In  all  low 
latitudes  the  sun  rides  high  in  the  heavens  at  midday, 
even  in  winter,  and  thus  melts  snow  fairly  effectively  in 
clear  weather.  This  is  highly  unfavorable  to  glaciation. 
The  inblowing  southern  monsoons  bring  all  their  mois- 
ture in  midsummer  at  just  the  time  when  it  is  least  effec- 
tive in  producing  snow.  Conditions  similar  to  those  now 
prevailing  in  the  Himalayas  must  accompany  any  great 
uplift  of  the  lands  which  produces  high  mountains  and 
large  continents  in  subtropical  and  middle  latitudes. 
Hence,  uplift  alone  cannot  account  for  extensive  glacia- 
tion in  subtropical  latitudes  during  the  Permian  and 
Proterozoic. 

The  assumption  of  a  great  general  lowering  of  tem- 
perature is  also  not  adequate  to  explain  glaciation  in 
subtropical  latitudes.  In  the  first  place  this  would  require 
a  lowering  of  many  degrees, — far  more  than  in  the  Pleis- 
tocene glacial  period.  The  marine  fossils  of  the  Permian, 
however,  do  not  indicate  any  such  condition.  In  the 
second  place,  if  the  lands  were  widespread  as  they  ap- 
pear to  have  been  in  the  Permian,  a  general  lowering  of 
temperature  would  dimmish  rather  than  increase  the 
present  slight  efficiency  of  the  monsoons  in  producing 
glaciation.  Monsoons  depend  upon  the  difference  between 
the  temperatures  of  land  and  water.  If  the  general  tem- 
perature were  lowered,  the  reduction  would  be  much  less 
pronounced  on  the  oceans  than  on  the  lands,  for  water 
tends  to  preserve  a  uniform  temperature,  not  only  be- 
cause of  its  mobility,  but  because  of  the  large  amount  of 
heat  given  out  when  freezing  takes  place,  or  consumed  in 
evaporation.  Hence  the  general  lowering  of  temperature 


SOME  PROBLEMS  OF  GLACIAL  PERIODS      147 

would  make  the  contrast  between  continents  and  oceans 
less  than  at  present  in  summer,  for  the  land  temperature 
would  be  brought  toward  that  of  the  ocean.  This  would 
diminish  the  strength  of  the  inblowing  summer  mon- 
soons and  thus  cut  off  part  of  the  supply  of  moisture. 
Evidence  that  this  actually  happened  in  the  cold  four- 
teenth century  has  already  been  given  in  Chapter  VI. 
On  the  other  hand,  in  winter  the  lands  would  be  much 
colder  than  now  and  the  oceans  only  a  little  colder,  so 
that  the  dry  outblowing  monsoons  of  the  cold  season 
would  increase  in  strength  and  would  also  last  longer 
than  at  present.  In  addition  to  all  this,  the  mere  fact  of 
low  temperature  would  mean  a  general  reduction  in  the 
amount  of  water  vapor  in  the  air.  Thus,  from  almost 
every  point  of  view  a  mere  lowering  of  temperature 
seems  to  be  ruled  out  as  a  cause  of  Permian  glaciation. 
Moreover,  if  the  Permian  or  Proterozoic  glacial  periods 
were  so  cold  that  the  lands  above  latitude  30°  were  snow- 
covered  most  of  the  time,  the  normal  surface  winds  in 
subtropical  latitudes  would  be  largely  equatorward,  just 
as  the  winter  monsoons  now  are.  Hence  little  or  no  mois- 
ture would  be  available  to  feed  the  snowfields  which  give 
rise  to  the  glaciers. 

It  has  been  assumed  by  Marsden  Manson  and  others 
that  increased  general  cloudiness  would  account  for  the 
subtropical  glaciation  of  the  Permian  and  Proterozoic. 
Granting  for  the  moment  that  there  could  be  universal 
persistent  cloudiness,  this  would  not  prevent  or  counter- 
act the  outblowing  anti-cyclonic  winds  so  characteristic 
of  great  snowfields.  Therefore,  under  the  hypothesis  of 
general  cloudiness  there  would  be  no  supply  of  moisture 
to  cause  glaciation  in  low  latitudes.  Indeed,  persistent 
cloudiness  in  all  higher  latitudes  would  apparently  de- 
prive the  Himalayas  of  most  of  their  present  moisture, 


148  CLIMATIC  CHANGES 

for  the  interior  of  Asia  would  not  become  hot  in  summer 
and  no  inblowing  monsoons  would  develop.  In  fact,  winds 
of  all  kinds  would  seemingly  be  scarce,  for  they  arise 
almost  wholly  from  contrasts  of  temperature  and  hence 
of  atmospheric  pressure.  The  only  way  to  get  winds  and 
hence  precipitation  would  be  to  invoke  some  other  agency, 
such  as  cyclonic  storms,  but  that  would  be  a  departure 
from  the  supposition  that  glaciation  arose  from  cloudi- 
ness. 

Let  us  now  inquire  how  the  cyclonic  hypothesis 
accounts  for  glaciation  in  low  latitudes.  We  will  first 
consider  the  terrestrial  conditions  in  the  early  Permian, 
the  last  period  of  glaciation  in  such  latitudes.  Geologists 
are  almost  universally  agreed  that  the  lands  were  excep- 
tionally extensive  and  also  high,  especially  in  low  lati- 
tudes. One  evidence  of  this  is  the  presence  of  abundant 
conglomerates  composed  of  great  boulders.  It  is  also 
probable  that  the  carbon  dioxide  in  the  air  during  the 
early  Permian  had  been  reduced  to  a  minimum  by  the 
extraordinary  amount  of  coal  formed  during  the  preced- 
ing period.  This  would  tend  to  produce  low  temperature 
and  thus  make  the  conditions  favorable  for  glaciation  as 
soon  as  an  accentuation  of  solar  activity  caused  unusual 
storminess.  If  the  storminess  became  extreme  when  ter- 
restrial conditions  were  thus  universally  favorable  to 
glaciation,  it  would  presumably  produce  glaciation  in  low 
latitudes.  Numerous  and  intense  tropical  cyclones  would 
carry  a  vast  amount  of  moisture  out  of  the  tropics,  just 
as  now  happens  when  the  sun  is  active,  but  on  a  far 
larger  scale.  The  moisture  would  be  precipitated  on  the 
equatorward  slopes  of  the  subtropical  mountain  ranges. 
At  high  elevations  this  precipitation  would  be  in  the  form 
of  snow  even  in  summer.  Tropical  cyclones,  however,  as 
is  shown  in  Earth  and  Sun,  occur  in  the  autumn  and 


SOME  PROBLEMS  OF  GLACIAL  PERIODS      149 

winter  as  well  as  in  summer.  For  example,  in  the  Bay  of 
Bengal  the  number  recorded  in  October  is  fifty,  the 
largest  for  any  month;  while  in  November  it  is  thirty- 
four,  and  December  fourteen  as  compared  with  an  aver- 
age of  forty-two  for  the  months  of  July  to  September. 
From  January  to  March,  when  sunspot  numbers  aver- 
aged more  than  forty,  the  number  of  tropical  hurricanes 
was  143  per  cent  greater  than  when  the  sunspot  numbers 
averaged  below  forty.  During  the  months  from  April  to 
June,  which  also  would  be  times  of  considerable  snowy 
precipitation,  tropical  hurricanes  averaged  58  per  cent 
more  numerous  with  sunspot  numbers  above  forty  than 
with  numbers  below  forty,  while  from  July  to  September 
the  difference  amounted  to  23  per  cent.  Even  at  this 
season  some  snow  falls  on  the  higher  slopes,  while  the 
increased  cloudiness  due  to  numerous  storms  also  tends 
to  preserve  the  snow.  Thus  a  great  increase  in  the  fre- 
quency of  sunspots  is  accompanied  by  increased  intensity 
of  tropical  hurricanes,  especially  in  the  cooler  autumn  and 
spring  months,  and  results  not  only  in  a  greater  accumu- 
lation of  snow  but  in  a  decrease  in  the  melting  of  the 
snow  because  of  more  abundant  clouds.  At  such  times  as 
the  Permian,  the  general  low  temperature  due  to  rapid 
convection  and  to  the  scarcity  of  carbon  dioxide  pre- 
sumably joined  with  the  extension  of  the  lands  in  pro- 
ducing great  high-pressure  areas  over  the  lands  in  middle 
latitudes  during  the  winters,  and  thus  caused  the  more 
northern,  or  mid-latitude  type  of  cyclonic  storms  to  be 
shifted  to  the  equatorward  side  of  the  continents  at  that 
season.  This  would  cause  an  increase  of  precipitation  in 
winter  as  well  as  during  the  months  when  tropical  hurri- 
canes abound.  Many  other  circumstances  would  cooper- 
ate to  produce  a  similar  result.  For  example,  the  general 
low  temperature  would  cause  the  sea  to  be  covered  with 


|U 


150  CLIMATIC  CHANGES 

ice  in  lower  latitudes  than  now,  and  would  help  to  create 
high-pressure  areas  in  middle  latitudes,  thus  driving  the 
storms  far  south.  If  the  sea  water  were  fresher  than  now, 
as  it  probably  was  to  a  notable  extent  in  the  Proterozoic 
and  perhaps  to  some  slight  extent  in  the  Permian,  the 
higher  freezing  point  would  also  further  the  extension 
of  the  ice  and  help  to  keep  the  storms  away  from  high 
latitudes.  If  to  this  there  is  added  a  distribution  of  land 
and  sea  such  that  the  volume  of  the  warm  ocean  currents 
flowing  from  low  to  high  latitudes  was  diminished,  as 
appears  to  have  been  the  case,  there  seems  to  be  no  diffi- 
culty in  explaining  the  subtropical  location  of  the  main 
glaciation  in  both  the  Permian  and  the  Proterozoic.  An 
increase  of  storminess  seems  to  be  the  key  to  the  whole 
situation. 

One  other  possibility  may  be  mentioned,  although  little 
stress  should  be  laid  on  it.  In  Earth  and  Sun  it  has  been 
shown  that  the  main  storm  track  in  both  the  northern 
and  southern  hemispheres  is  not  concentric  with  the 
geographical  poles.  Both  tracks  are  roughly  concentric 
with  the  corresponding  magnetic  poles,  a  fact  which  may 
be  important  in  connection  with  the  hypothesis  of  an  elec- 
trical effect  of  the  sun  upon  terrestrial  storminess.  The 
magnetic  poles  are  known  to  wander  considerably.  Such 
wandering  gives  rise  to  variations  in  the  direction  of 
the  magnetic  needle  from  year  to  year.  In  1815  the  com- 
pass in  England  pointed  24y2°  W.  of  N.  and  in  1906 
17°  45'  W.  Such  a  variation  seems  to  mean  a  change  of 
many  miles  in  the  location  of  the  north  magnetic  pole. 
Certain  changes  in  the  daily  march  of  electromagnetic 
phenomena  over  the  oceans  have  led  Bauer  and  his  asso- 
ciates to  suggest  that  the  magnetic  poles  may  even  be 
subject  to  a  slight  daily  movement  in  response  to  the 
changes  in  the  relative  positions  of  the  earth  and  sun. 


SOME  PROBLEMS  OF  GLACIAL  PERIODS      151 

Thus  there  seems  to  be  a  possibility  that  a  pronounced 
change  in  the  location  of  the  magnetic  pole  in  Permian 
times,  for  example,  may  have  had  some  connection  with 
a  shifting  in  fhe  location  of  the  belt  of  storms.  It  must  be 
clearly  understood  that  there  is  as  yet  no  evidence  of  any 
such  change,  and  the  matter  is  introduced  merely  to  call 
attention  to  a  possible  line  of  investigation. 

Any  hypothesis  of  Permian  and  Proterozoic  glaciation 
must  explain  not  only  the  glaciation  of  low  latitudes  but 
the  lack  of  glaciation  and  the  accumulation  of  red  desert 
beds  in  high  latitudes.  The  facts  already  presented  seem 
to  explain  this.  Glaciation  could  not  occur  extensively  in 
high  latitudes  partly  because  during  most  of  the  year  the 
air  was  too  cold  to  hold  much  moisture,  but  still  more 
because  the  winds  for  the  most  part  must  have  blown 
outward  from  the  cold  northern  areas  and  the  cyclonic 
storm  belt  was  pushed  out  of  high  latitudes.  Because  of 
these  conditions  precipitation  was  apparently  limited  to 
a  relatively  small  number  of  storms  during  the  summer. 
Hence  great  desert  areas  must  have  prevailed  at  high 
latitudes.  Great  aridity  now  prevails  north  of  the  Hima- 
layas and  related  ranges,  and  red  beds  are  accumulating 
in  the  centers  of  the  great  deserts,  such  as  those  of  the 
Tarim  Basin  and  the  Transcaspian.  The  redness  is  not 
due  to  the  original  character  of  the  rock,  but  to  intense 
oxidation,  as  appears  from  the  fact  that  along  the  edges 
of  the  desert  and  wherever  occasional  floods  carry  sedi- 
ment far  out  into  the  midst  of  the  sand,  the  material  has 
the  ordinary  brownish  shades.  As  soon  as  one  goes  out 
into  the  places  where  the  sand  has  been  exposed  to  the  air 
for  a  long  time,  however,  it  becomes  pink,  and  then  red. 
Such  conditions  may  have  given  rise  to  the  high  degree 
of  oxidation  in  the  famous  Permian  red  beds.  If  the  air 
of  the  early  Permian  contained  an  unusual  percentage  of 


152  CLIMATIC  CHANGES 

oxygen  because  of  the  release  of  that  gas  by  the  great 
plant  beds  which  formed  coal  in  the  preceding  era,  as 
Chamberlin  has  thought  probable,  the  tendency  to  pro- 
duce red  beds  would  be  still  further  increased. 

It  must  not  be  supposed,  however,  that  these  condi- 
tions would  absolutely  limit  glaciation  to  subtropical 
latitudes.  The  presence  of  early  Permian  glaciation  in 
North  America  at  Boston  and  in  Alaska  and  in  the  Falk- 
land Islands  of  the  South  Atlantic  Ocean  proves  that  at 
least  locally  there  was  sufficient  moisture  to  form  glaciers 
near  the  coast  in  relatively  high  latitudes.  The  possibility 
of  this  would  depend  entirely  upon  the  form  of  the  lands 
and  the  consequent  course  of  ocean  currents.  Even  in 
those  high  latitudes  cyclonic  storms  would  occur  unless 
they  were  kept  out  by  conditions  of  pressure  such  as  have 
been  described  above. 

The  marine  faunas  of  Permian  age  in  high  latitudes 
have  been  interpreted  as  indicating  mild  oceanic  tempera- 
tures. This  is  a  point  which  requires  further  investiga- 
tion. Warm  oceans  during  times  of  slight  solar  activity 
are  a  necessary  consequence  of  the  cyclonic  hypothesis, 
as  will  appear  later.  The  present  cold  oceans  seem  to  be 
the  expectable  result  of  the  Pleistocene  glaciation  and  of 
the  present  relatively  disturbed  condition  of  the  sun.  If 
a  sudden  disturbance  threw  the  solar  atmosphere  into 
violent  commotion  within  a  few  thousand  years  during 
Permian  times,  glaciation  might  occur  as  described  above, 
while  the  oceans  were  still  warm.  In  fact  their  warmth 
would  increase  evaporation  while  the  violent  cyclonic 
storms  and  high  winds  would  cause  heavy  rain  and  keep 
the  air  cool  by  constantly  raising  it  to  high  levels  where 
it  would  rapidly  radiate  its  heat  into  space. 

Nevertheless  it  is  not  yet  possible  to  determine  how 
warm  the  oceans  were  at  the  actual  time  of  the  Permian 


SOME  PROBLEMS  OF  GLACIAL  PERIODS      153 

glaciation.  Some  faunas  formerly  reported  as  Permian 
are  now  known  to  be  considerably  older.  Moreover,  others 
of  undoubted  Permian  age  are  probably  not  strictly  con- 
temporaneous with  the  glaciation.  So  far  back  in  the 
geological  record  it  is  very  doubtful  whether  we  can  date 
fossils  within  the  limits  of  say  100,000  years.  Yet  a  dif- 
ference  of  100,000  years  would  be  more  than  enough  to 
allow  the  fossils  to  have  lived  either  before  or  after  the 
glaciation,  or  in  an  inter-glacial  epoch.  One  such  epoch 
is  known  to  have  occurred  and  nine  others  are  suggested 
by  the  inter-stratification  of  glacial  till  and  marine  sedi- 
ments in  eastern  Australia.  The  warm  currents  which 
would  flow  poleward  in  inter-glacial  epochs  must  have 
favored  a  prompt  reintroduction  of  marine  faunas  driven 
out  during  times  of  glaciation.  Taken  all  and  all,  the 
Permian  glaciation  seems  to  be  accounted  for  by  the 
cyclonic  hypothesis  quite  as  well  as  does  the  Pleistocene. 
In  both  these  cases,  as  well  as  in  the  various  pulsations 
of  historic  times,  it  seems  to  be  necessary  merely  to  mag- 
nify what  is  happening  today  in  order  to  reproduce  the 
conditions  which  prevailed  in  the  past.  If  the  conditions 
which  now  prevail  at  times  of  sunspot  minima  were  mag- 
nified, they  would  give  the  mild  conditions  of  inter-glacial 
epochs  and  similar  periods.  If  the  conditions  which  now 
prevail  at  times  of  sunspot  maxima  are  magnified  a  little 
they  seem  to  produce  periods  of  climatic  stress  such  as 
those  of  the  fourteenth  century.  If  they  are  magnified 
still  more  the  result  is  apparently  glacial  epochs  like 
those  of  the  Pleistocene,  and  if  they  are  magnified  to  a 
still  greater  extent,  the  result  is  Permian  or  Proterozoic 
glaciation.  Other  factors  must  indeed  be  favorable,  for 
climatic  changes  are  highly  complex  and  are  unques- 
tionably due  to  a  combination  of  circumstances.  The  point 
which  is  chiefly  emphasized  in  this  book  is  that  among 


154  CLIMATIC  CHANGES 

those  several  circumstances,  changes  in  cyclonic  storms 
due  apparently  to  activity  of  the  sun's  atmosphere  must 
always  be  reckoned. 


CHAPTER  IX 
THE  ORIGIN  OF  LOESS 

ONE  of  the  most  remarkable  formations  associ- 
ated with  glacial  deposits  consists  of  vast  sheets 
of  the  fine-grained,  yellowish,  wind-blown  ma- 
terial called  loess.  Somewhat  peculiar  climatic  condi- 
tions evidently  prevailed  when  it  was  formed.  At  present 
similar  deposits  are  being  laid  down  only  near  the  lee- 
ward margin  of  great  deserts.  The  famous  loess  deposits 
of  China  in  the  lee  of  the  Desert  of  Gobi  are  examples. 
During  the  Pleistocene  period,  however,  loess  accumu- 
lated in  a  broad  zone  along  the  margin  of  the  ice  sheet 
at  its  maximum  extent.  In  the  Old  World  it  extended 
from  France  across  Germany  and  through  the  Black 
Earth  region  of  Eussia  into  Siberia.  In  the  New  World 
a  still  larger  area  is  loess-covered.  In  the  Mississippi 
Valley,  tens  of  thousands  of  square  miles  are  mantled  by 
a  layer  exceeding  twenty  feet  in  thickness  and  in  many 
places  approaching  a  hundred  feet.  Neither  the  North 
American  nor  the  European  deposits  are  associated  with 
a  desert.  Indeed,  loess  is  lacking  in  the  western  and 
drier  parts  of  the  great  plains  and  is  best  developed  in 
the  well-watered  states  of  Iowa,  Illinois,  and  Missouri. 
Part  of  the  loess  overlies  the  non-glacial  materials  of  the 
great  central  plain,  but  the  northern  portions  overlie  the 
drift  deposits  of  the  first  three  glaciations.  A  few  traces 
of  loess  are  associated  with  the  Kansan  and  Illinoian, 
the  second  and  third  glaciations,  but  most  of  the  Ameri- 


156  CLIMATIC  CHANGES 

can  loess  appears  to  have  been  formed  at  approximately 
the  time  of  the  lowan  or  fourth  glaciation,  while  only  a 
little  overlies  the  drift  sheets  of  the  Wisconsin  age.  The 
loess  is  thickest  near  the  margin  of  the  lowan  till  sheet 
and  thins  progressively  both  north  and  south.  The 
thinning  southward  is  abrupt  along  the  stream  divides, 
but  very  gradual  along  the  larger  valleys.  Indeed,  loess  is 
abundant  along  the  bluffs  of  the  Mississippi,  especially 
the  east  bluff,  almost  to  the  Gulf  of  Mexico.1 

It  is  now  generally  agreed  that  all  typical  loess  is  wind 
blown.  There  is  still  much  question,  however,  as  to  its 
time  of  origin,  and  thus  indirectly  as  to  its  climatic  im- 
plications. Several  American  and  European  students 
have  thought  that  the  loess  dates  from  inter-glacial  times. 
On  the  other  hand,  Penck  has  concluded  that  the  loess 
was  formed  shortly  before  the  commencement  of  the 
glacial  epochs ;  while  many  American  geologists  hold  that 
the  loess  accumulated  while  the  ice  sheets  were  at  ap- 
proximately their  maximum  size.  W.  J.  McGee,  Cham- 
berlin  and  Salisbury,  Keyes,  and  others  lean  toward  this 
view.  In  this  chapter  the  hypothesis  is  advanced  that  it 
was  formed  at  the  one  other  possible  time,  namely,  imme- 
diately following  the  retreat  of  the  ice. 

These  four  hypotheses  as  to  the  time  of  origin  of  loess 
imply  the  following  differences  in  its  climatic  relations. 
If  loess  was  formed  during  typical  inter-glacial  epochs, 
or  toward  the  close  of  such  epochs,  profound  general 
aridity  must  seemingly  have  prevailed  in  order  to  kill 
off  the  vegetation  and  thus  enable  the  wind  to  pick  up 
sufficient  dust.  If  the  loess  was  formed  during  times  of 
extreme  glaciation  when  the  glaciers  were  supplying 
large  quantities  of  fine  material  to  outflowing  streams, 
less  aridity  would  be  required,  but  there  must  have  been 

i  Chamberlin  and  Salisbury:  Geology,  1906,  Vol.  Ill,  pp.  405-412. 


THE  ORIGIN  OF  LOESS  157 

sharp  contrasts  between  wet  seasons  in  summer  when 
the  snow  was  melting  and  dry  seasons  in  winter  when 
the  storms  were  forced  far  south  by  the  glacial  high  pres- 
sure. Alternate  floods  and  droughts  would  thus  affect 
broad  areas  along  the  streams.  Hence  arises  the  hypothe- 
sis that  the  wind  obtained  the  loess  from  the  flood  plains 
of  streams  at  times  of  maximum  glaciation.  If  the  loess 
was  formed  during  the  rapid  retreat  of  the  ice,  alternate 
summer  floods  and  winter  droughts  would  still  prevail, 
but  much  material  could  also  be  obtained  by  the  winds 
not  only  from  flood  plains,  but  also  from  the  deposits 
exposed  by  the  melting  of  the  ice  and  not  yet  covered  by 
vegetation. 

The  evidence  for  and  against  the  several  hypotheses 
may  be  stated  briefly.  In  support  of  the  hypothesis  of  the 
inter-glacial  origin  of  loess,  Shimek  and  others  state  that 
the  glacial  drift  which  lies  beneath  the  loess  commonly 
gives  evidence  that  some  time  elapsed  between  the  dis- 
appearance of  the  ice  and  the  deposition  of  the  loess.  For 
example,  abundant  shells  of  land  snails  in  the  loess  are 
not  of  the  sort  now  found  in  colder  regions,  but  resemble 
those  found  in  the  drier  regions.  It  is  probable  that  if 
they  represented  a  glacial  epoch  they  would  be  depauper- 
ated by  the  cold  as  are  the  snails  of  far  northern  regions. 
The  gravel  pavement  discussed  below  seems  to  be  strong 
evidence  of  erosion  between  the  retreat  of  the  ice  and 
the  deposition  of  the  loess. 

Turning  to  the  second  hypothesis,  namely,  that  the 
loess  accumulated  near  the  close  of  the  inter-glacial  epoch 
rather  than  in  the  midst  of  it,  we  may  follow  Penck.  The 
mammalian  fossils  seem  to  him  to  prove  that  the  loess 
was  formed  while  boreal  animals  occupied  the  region,  for 
they  include  remains  of  the  hairy  mammoth,  woolly  rhi- 
noceros, and  reindeer.  On  the  other  hand,  the  typical 


158  CLIMATIC  CHANGES 

inter-glacial  beds  not  far  away  yield  remains  of  species 
characteristic  of  milder  climates,  such  as  the  elephant, 
the  smaller  rhinoceros,  and  the  deer.  In  connection  with 
these  facts  it  should  be  noted  that  occasional  remains  of 
tundra  vegetation  and  of  trees  are  found  beneath  the 
loess,  while  in  the  loess  itself  certain  steppe  animals, 
such  as  the  common  gopher  or  spermaphyl,  are  found. 
Penck  interprets  this  as  indicating  a  progressive  desicca- 
tion culminating  just  before  the  oncoming  of  the  next  ice 
sheet. 

The  evidence  advanced  in  favor  of  the  hypothesis  that 
the  loess  was  formed  when  glaciation  was  near  its  maxi- 
mum includes  the  fact  that  if  the  loess  does  not  represent 
the  outwash  from  the  lowan  ice,  there  is  little  else  that 
does,  and  presumably  there  must  have  been  outwash. 
Also  the  distribution  of  loess  along  the  margins  of 
streams  suggests  that  much  of  the  material  came  from 
the  flood  plains  of  overloaded  streams  flowing  from  the 
melting  ice. 

Although  there  are  some  points  in  favor  of  the  hy- 
pothesis that  the  loess  originated  (1)  in  strictly  inter- 
glacial  times,  (2)  at  the  end  of  inter-glacial  epochs,  and 
(3)  at  times  of  full  glaciation,  each  hypothesis  is  much 
weakened  by  evidence  that  supports  the  others.  The  evi- 
dence of  boreal  animals  seems  to  disprove  the  hypothesis 
that  the  loess  was  formed  in  the  middle  of  a  mild  inter- 
glacial  epoch.  On  the  other  hand,  Penck 's  hypothesis  as 
to  loess  at  the  end  of  inter-glacial  times  fails  to  account 
for  certain  characteristics  of  the  lowest  part  of  the  loess 
deposits  and  of  the  underlying  topography.  Instead  of 
normal  valleys  and  consequent  prompt  drainage  such  as 
ought  to  have  developed  before  the  end  of  a  long  inter- 
glacial  epoch,  the  surface  on  which  the  loess  lies  shows 
many  undrained  depressions.  Some  of  these  can  be  seen 


THE  ORIGIN  OF  LOESS  159 

in  exposed  banks,  while  many  more  are  inferred  from  the 
presence  of  shells  of  pond  snails  here  and  there  in  the 
overlying  loess.  The  pond  snails  presumably  lived  in 
shallow  pools  occupying  depressions  in  the  uneven  sur- 
face left  by  the  ice.  Another  reason  for  questioning 
whether  the  loess  was  formed  at  the  end  of  an  inter- 
glacial  epoch  is  that  this  hypothesis  does  not  provide  a 
reasonable  origin  for  the  material  which  composes  the 
loess.  Near  the  Alps  where  the  loess  deposits  are  small 
and  where  glaciers  probably  persisted  in  the  inter-glacial 
epochs  and  thus  supplied  flood  plain  material  in  large 
quantities,  this  does  not  appear  important.  In  the  broad 
upper  Mississippi  Basin,  however,  and  also  in  the  Black 
Earth  region  of  Eussia  there  seems  to  be  no  way  to  get 
the  large  body  of  material  composing  the  loess  except  by 
assuming  the  existence  of  great  deserts  to  windward. 
But  there  seems  to  be  little  or  no  evidence  of  such  deserts 
where  they  could  be  effective.  The  mineralogical  char- 
acter of  the  loess  of  lowan  age  proves  that  the  material 
came  from  granitic  rocks,  such  as  formed  a  large  part  of 
the  drift.  The  nearest  extensive  outcrops  of  granite  are 
in  the  southwestern  part  of  the  United  States,  nearly  a 
thousand  miles  from  Iowa  and  Illinois.  But  the  loess  is 
thickest  near  the  ice  margin  and  thins  toward  the  south- 
west and  in  other  directions,  whereas  if  its  source  were 
the  southwestern  desert,  its  maximum  thickness  would 
probably  be  near  the  margin  of  the  desert. 

The  evidence  cited  above  seems  inconsistent  not  only 
with  the  hypothesis  that  the  loess  was  formed  at  the  end 
of  an  inter-glacial  epoch,  but  also  with  the  idea  that  it 
originated  at  times  of  maximum  glaciation  either  from 
river-borne  sediments  or  from  any  other  source.  A 
further  and  more  convincing  reason  for  this  last  con- 
clusion is  the  probability  and  almost  the  certainty  that 


160  CLIMATIC  CHANGES 

when  the  ice  advanced,  its  front  lay  close  to  areas  where 
the  vegetation  was  not  much  thinner  than  that  which 
today  prevails  under  similar  climatic  conditions.  If  the 
average  temperature  of  glacial  maxima  was  only  6°C. 
lower  than  that  of  today,  the  conditions  just  beyond  the 
ice  front  when  it  was  in  the  loess  region  from  southern 
Illinois  to  Minnesota  would  have  been  like  those  now  pre- 
vailing in  Canada  from  New  Brunswick  to  Winnipeg. 
The  vegetation  there  is  quite  different  from  the  grassy, 
semi-arid  vegetation  of  which  evidence  is  found  in  the 
loess.  The  roots  and  stalks  of  such  grassy  vegetation  are 
generally  agreed  to  have  helped  produce  the  columnar 
structure  which  enables  the  loess  to  stand  with  almost 
vertical  surfaces. 

We  are  now  ready  to  consider  the  probability  that  loess 
accumulated  mainly  during  the  retreat  of  the  ice.  Such  a 
retreat  exposed  a  zone  of  drift  to  the  outflowing  glacial 
winds.  Most  glacial  hypotheses,  such  as  that  of  uplift, 
or  depleted  carbon  dioxide,  call  for  a  gradual  retreat 
of  the  ice  scarcely  faster  than  the  vegetation  could  ad- 
vance into  the  abandoned  area.  Under  the  solar-cyclonic 
hypothesis,  on  the  other  hand,  the  climatic  changes  may 
have  been  sudden  and  hence  the  retreat  of  the  ice  may 
have  been  much  more  rapid  than  the  advance  of  vegeta- 
tion. Now  wind-blown  materials  are  derived  from  places 
where  vegetation  is  scanty.  Scanty  vegetation  on  good 
soil,  it  is  true,  is  usually  due  to  aridity,  but  may  also 
result  because  the  time  since  the  soil  was  exposed  to  the 
air  has  not  been  long  enough  for  the  soil  to  be  sufficiently 
weathered  to  support  vegetation.  Even  when  weathering 
has  had  full  opportunity,  as  when  sand  bars,  mud  flats, 
and  flood  plains  are  exposed,  vegetation  takes  root  only 
slowly.  Moreover,  storms  and  violent  winds  may  prevent 
the  spread  of  vegetation,  as  is  seen  on  sandy  beaches  even 


THE  ORIGIN  OF  LOESS  161 

in  distinctly  humid  regions  like  New  Jersey  and  Den- 
mark. Thus  it  appears  that  unless  the  retreat  of  the  ice 
were  as  slow  as  the  advance  of  vegetation,  a  barren  area 
of  more  or  less  width  must  have  bordered  the  retreating 
ice  and  formed  an  ideal  source  of  loess. 

Several  other  lines  of  evidence  seemingly  support  the 
conclusion  that  the  loess  was  formed  during  the  retreat 
of  the  ice.  For  example,  Shimek,  who  has  made  almost 
a  lifelong  study  of  the  lowan  loess,  emphasizes  the  fact 
that  there  is  often  an  accumulation  of  stones  and  pebbles 
at  its  base.  This  suggests  that  the  underlying  till  was 
eroded  before  the  loess  was  deposited  upon  it.  The  first 
reaction  of  most  students  is  to  assume  that  of  course 
this  was  due  to  running  water.  That  is  possible  in  many 
cases,  but  by  no  means  in  all.  So  widespread  a  sheet  of 
gravel  could  not  be  deposited  by  streams  without  destroy- 
ing the  irregular  basins  and  hollows  of  which  we  have 
seen  evidence  where  the  loess  lies  on  glacial  deposits.  On 
the  other  hand,  the  wind  is  competent  to  produce  a  simi- 
lar gravel  pavement  without  disturbing  the  old  topog- 
raphy. " Desert  pavements"  are  a  notable  feature  in  most 
deserts.  On  the  edges  of  an  ice  sheet,  as  Hobbs  has  made 
us  realize,  the  commonest  winds  are  outward.  They  often 
attain  a  velocity  of  eighty  miles  an  hour  in  Antarctica 
and  Greenland.  Such  winds,  however,  usually  decline 
rapidly  in  velocity  only  a  few  score  miles  from  the  ice. 
Thus  their  effect  would  be  to  produce  rapid  erosion 
of  the  freshly  bared  surface  near  the  retreating  ice. 
The  pebbles  would  be  left  behind  as  a  pavement,  while 
sand  and  then  loess  would  be  deposited  farther  from  the 
ice  where  the  winds  were  weaker  and  where  vegetation 
was  beginning  to  take  root.  Such  a  decrease  in  wind 
velocity  may  explain  the  occasional  vertical  gradation 
from  gravel  through  sand  to  coarse  loess  and  then  to 


162  CLIMATIC  CHANGES 

normal  fine  loess.  As  the  ice  sheet  retreated  the  wind  in 
any  given  place  would  gradually  become  less  violent. 
As  the  ice  continued  to  retreat  the  area  where  loess  was 
deposited  would  follow  at  a  distance,  and  thus  each  part 
of  the  gravel  pavement  would  in  turn  be  covered  with  the 
loess. 

The  hypothesis  that  loess  is  deposited  while  the  ice  is 
retreating  is  in  accord  with  many  other  lines  of  evidence. 
For  example,  it  accords  with  the  boreal  character  of  the 
mammal  remains  as  described  above.  Again,  the  advance 
of  vegetation  into  the  barren  zone  along  the  front  of  the 
ice  would  be  delayed  by  the  strong  outblowing  winds. 
The  common  pioneer  plants  depend  largely  on  the  wind 
for  the  distribution  of  their  seeds,  but  the  glacial  winds 
would  carry  them  away  from  the  ice  rather  than  toward 
it.  The  glacial  winds  discourage  the  advance  of  vegeta- 
tion in  another  way,  for  they  are  drying  winds,  as  are 
almost  all  winds  blowing  from  a  colder  to  a  warmer 
region.  The  fact  that  remains  of  trees  sometimes  occur 
at  the  bottom  of  the  loess  probably  means  that  the  depo- 
sition of  loess  extended  into  the  forests  which  almost 
certainly  persisted  not  far  from  the  ice.  This  seems  more 
likely  than  that  a  period  of  severe  aridity  before  the  ad- 
vance of  the  ice  killed  the  trees  and  made  a  steppe  or 
desert.  Penck's  chief  argument  in  favor  of  the  formation 
of  loess  before  the  advance  of  the  ice  rather  than  after, 
is  that  since  loess  is  lacking  upon  the  youngest  drift  sheet 
in  Europe  it  must  have  been  formed  before  rather  than 
after  the  last  or  Wiirm  advance  of  the  ice.  This  breaks 
down  on  two  counts.  First,  on  the  corresponding  (Wis- 
consin) drift  sheet  in  America,  loess  is  present, — in  small 
quantities  to  be  sure,  but  unmistakably  present.  Second, 
there  is  no  reason  to  assume  that  conditions  were  identi- 
cal at  each  advance  and  retreat  of  the  ice.  Indeed,  the 


THE  ORIGIN  OF  LOESS  163 

fact  that  in  Europe,  as  in  the  United  States,  nearly  all 
the  loess  was  formed  at  one  time,  and  only  a  little  is  asso- 
ciated with  the  other  ice  advances,  points  clearly  against 
Penck's  fundamental  assumption  that  the  accumulation 
of  loess  was  due  to  the  approach  of  a  cold  climate. 

Having  seen  that  the  loess  was  probably  formed  during 
the  retreat  of  the  ice,  we  are  now  ready  to  inquire  what 
conditions  the  cyclonic  hypothesis  would  postulate  in  the 
loess  areas  during  the  various  stages  of  a  glacial  cycle. 
Fig.  2,  in  Chapter  IV,  gives  the  best  idea  of  what  would 
apparently  happen  in  North  America,  and  events  in  Europe 
would  presumably  be  similar.  During  the  nine  maximum 
years  on  which  Fig.  2  is  based  the  sunspot  numbers  aver- 
aged seventy,  while  during  the  nine  minimum  years  they 
averaged  less  than  five.  It  seems  fair  to  suppose  that  the 
maximum  years  represent  the  average  conditions  which 
prevailed  in  the  past  at  times  when  the  sun  was  in  a 
median  stage  between  the  full  activity  which  led  to  glacia- 
tion  and  the  mild  activity  of  the  minimum  years  which 
appear  to  represent  inter-glacial  conditions.  This  would 
mean  that  when  a  glacial  period  was  approaching,  but 
before  an  ice  sheet  had  accumulated  to  any  great  extent, 
a  crescent-shaped  strip  from  Montana  through  Illinois  to 
Maine  would  suffer  a  diminution  in  storminess  ranging 
up  to  60  per  cent  as  compared  with  inter-glacial  condi- 
tions. This  is  in  strong  contrast  with  an  increase  in 
storminess  amounting  to  75  or  even  100  per  cent  both  in 
the  boreal  storm  belt  in  Canada  and  in  the  subtropical 
belt  in  the  Southwest.  Such  a  decrease  in  storminess  in 
the  central  United  States  would  apparently  be  most 
noticeable  in  summer,  as  is  shown  in  Earth  and  Sun.  ' j 
Hence  it  would  have  a  maximum  effect  in  producing 
aridity.  This  would  favor  the  formation  of  loess,  but  it  is 
doubtful  whether  the  aridity  would  become  extreme 


164  CLIMATIC  CHANGES 

enough  to  explain  such  vast  deposits  as  are  found 
throughout  large  parts  of  the  Mississippi  Basin.  That 
would  demand  that  hundreds  of  thousands  of  square  miles 
should  become  almost  absolute  desert,  and  it  is  not  prob- 
able that  any  such  thing  occurred.  Nevertheless,  accord- 
ing to  the  cyclonic  hypothesis  the  period  immediately 
before  the  advent  of  the  ice  would  be  relatively  dry  in 
the  central  United  States,  and  to  that  extent  favorable  to 
the  work  of  the  wind. 

As  the  climatic  conditions  became  more  severe  and  the 
ice  sheet  expanded,  the  dryness  and  lack  of  storms  would 
apparently  diminish.  The  reason,  as  has  been  explained, 
would  be  the  gradual  pushing  of  the  storms  southward 
by  the  high-pressure  area  which  would  develop  over  the 
ice  sheet.  Thus  at  the  height  of  a  glacial  epoch  there 
would  apparently  be  great  storminess  in  the  area  where 
the  loess  is  found,  especially  in  summer.  Hence  the 
cyclonic  hypothesis  does  not  accord  with  the  idea  of  great 
deposition  of  loess  at  the  time  of  maximum  glaciation. 

Finally  we  come  to  the  time  when  the  ice  was  retreat- 
ing. We  have  already  seen  that  not  only  the  river  flood 
plains,  but  also  vast  areas  of  fresh  glacial  deposits  would 
be  exposed  to  the  winds,  and  would  remain  without  vege- 
tation for  a  longtime.  At  that  very  time  the  retreat  of 
the  ice  sheet  would  tend  to  permit  the  storms  to  follow 
paths  determined  by  the  degree  of  solar  activity,  in  place 
of  the  far  southerly  paths  to  which  the  high  atmospheric 
pressure  over  the  expanded  ice  sheet  had  previously 
forced  them.  In  other  words,  the  conditions  shown  in 
Fig.  2  would  tend  to  reappear  when  the  sun's  activity 
was  diminishing  and  the  ice  sheet  was  retreating,  just  as 
they  had  appeared  when  the  sun  was  becoming  more 
active  and  the  ice  sheet  was  advancing.  This  time,  how- 
ever, the  semi-arid  conditions  arising  from  the  scarcity 


THE  ORIGIN  OF  LOESS  165 

of  storms  would  prevail  in  a  region  of  glacial  deposits 
and  widely  spreading  river  deposits,  few  or  none  of 
which  would  be  covered  with  vegetation.  The  conditions 
would  be  almost  ideal  for  eolian  erosion  and  for  the 
transportation  of  loess  by  the  wind  to  areas  a  little  more 
remote  from  the  ice  where  grassy  vegetation  had  made  a 
start. 

The  cyclonic  hypothesis  also  seems  to  offer  a  satis- 
factory explanation  of  variations  in  the  amount  of  loess 
associated  with  the  several  glacial  epochs.  It  attributes 
these  to  differences  in  the  rate  of  disappearance  of  the 
ice,  which  in  turn  varied  with  the  rate  of  decline  of  solar 
activity  and  storminess.  This  is  supposed  to  be  the  reason 
why  the  lowan  loess  deposits  are  much  more  extensive 
than  those  of  the  other  epochs,  for  the  lowan  ice  sheet 
presumably  accomplished  part  of  its  retreat  much  more 
suddenly  than  the  other  ice  sheets.2  The  more  sudden  the 
retreat,  the  greater  the  barren  area  where  the  winds 
could  gather  fine  bits  of  dust.  Temporary  readvances  may 
also  have  been  so  distributed  and  of  such  intensity  that 
they  frequently  accentuated  the  condition  shown  in  Fig. 
2,  thus  making  the  central  United  States  dry  soon  after 
the  exposure  of  great  amounts  of  glacial  debris.  The 
closeness  with  which  the  cyclonic  hypothesis  accords  with 
the  facts  as  to  the  loess  is  one  of  the  pleasant  surprises  of 
the  hypothesis.  The  first  draft  of  Fig.  2  and  the  first  out- 
lines of  the  hypothesis  were  framed  without  thought  of 
the  loess.  Yet  so  far  as  can  now  be  seen,  both  agree 
closely  with  the  conditions  of  loess  formation. 

2  It  may  have  retreated  soon  after  reaching  its  maximum.  If  so,  the 
general  lack  of  thick  terminal  moraines  would  be  explained.  See  page  122. 


CHAPTER  X 
CAUSES  OF  MILD  GEOLOGICAL  CLIMATES 

IN  discussions  of  climate,  as  of  most  subjects,  a 
peculiar  psychological  phenomenon  is  observable. 
Everyone  sees  the  necessity  of  explaining  conditions 
different  from  those  that  now  exist,  but  few  realize  that 
present  conditions  may  be  abnormal,  and  that  they  need 
explanation  just  as  much  as  do  others.  Because  of  this 
tendency  glaciation  has  been  discussed  with  the  greatest 
fullness,  while  there  has  been  much  neglect  not  only  of 
the  periods  when  the  climate  of  the  earth  resembled  that 
of  the  present,  but  also  of  the  vastly  longer  periods  when 
it  was  even  milder  than  now. 

How  important  the  periods  of  mild  climate  have  been 
in  geological  times  may  be  judged  from  the  relative 
length  of  glacial  compared  with  inter-glacial  epochs,  and 
still  more  from  the  far  greater  relative  length  of  the  mild 
parts  of  periods  and  eras  when  compared  with  the  severe 
parts.  Recent  estimates  by  R.  T.  Chamberlin1  indicate 
that  according  to  the  consensus  of  opinion  among  geolo- 
gists the  average  inter-glacial  epoch  during  the  Pleisto- 
cene was  about  five  times  as  long  as  the  average  glacial 
epoch,  while  the  whole  of  a  given  glacial  epoch  averaged 
five  times  as  long  as  the  period  when  the  ice  was  at  a 
maximum.  Climatic  periods  far  milder,  longer,  and  more 
monotonous  than  any  inter-glacial  epoch  appear  repeat- 

i  Eollin  T.  Chamberlin :  Personal  Communication. 


CAUSES  OF  MILD  GEOLOGICAL  CLIMATES    167 

edly  during  the  course  of  geological  history.  Our  task  in 
this  chapter  is  to  explain  them. 

Knowlton2  has  done  geology  a  great  service  by  col- 
lecting the  evidence  as  to  the  mild  type  of  climate  which 
has  again  and  again  prevailed  in  the  past.  He  lays  special 
stress  on  botanical  evidence  since  that  pertains  to  the 
variable  atmosphere  of  the  lands,  and  hence  furnishes  a 
better  guide  than  does  the  evidence  of  animals  that  lived 
in  the  relatively  unchanging  water  of  the  oceans.  The 
nature  of  the  evidence  has  already  been  indicated  in 
various  parts  of  this  book.  It  includes  palms,  tree  ferns, 
and  a  host  of  other  plants  which  once  grew  in  regions 
which  are  now  much  too  cold  to  support  them.  With  this 
must  be  placed  the  abundant  reef-building  corals  and 
other  warmth-loving  marine  creatures  in  latitudes  now 
much  too  cold  for  them.  Of  a  piece  with  this  are  the  condi- 
tions of  inter-glacial  epochs  in  Europe,  for  example, 
when  elephants  and  hippopotamuses,  as  well  as  many 
species  of  plants  from  low  latitudes,  were  abundant. 
These  conditions  indicate  not  only  that  the  climate  was 
warmer  than  now,  but  that  the  contrast  from  season  to 
season  was  much  less.  Indeed,  Knowlton  goes  so  far  as 
to  say  that  "relative  uniformity,  mildness,  and  compara- 
tive equability  of  climate,  accompanied  by  high  humidity, 
have  prevailed  over  the  greater  part  of  the  earth,  extend- 
ing to,  or  into,  polar  circles,  during  the  greater  part  of 
geologic  time — since,  at  least,  the  Middle  Paleozoic.  This 
is  the  regular,  the  ordinary,  the  normal  condition. "... 
"By  many  it  is  thought  that  one  of  the  strongest  argu- 
ments against  a  gradually  cooling  globe  and  a  humid, 
non-zonally  disposed  climate  in  the  ages  before  the  Pleis- 
tocene is  the  discovery  of  evidences  of  glacial  action 

2  F.  H.  Knowlton :  Evolution  of  Geologic  Climates ;  Bull.  Geol.  Soc.  Am., 
Vol.  30,  1919,  pp.  499-566. 


168  CLIMATIC  CHANGES 

practically  throughout  the  entire  geologic  column. 
Hardly  less  than  a  dozen  of  these  are  now  known,  ranging 
in  age  from  Huronian  to  Eocene.  It  seems  to  be  a  very 
general  assumption  by  those  who  hold  this  view  that 
these  evidences  of  glacial  activities  are  to  be  classed  as 
ice  ages,  largely  comparable  in  effect  and  extent  to  the 
Pleistocene  refrigeration,  but  as  a  matter  of  fact  only 
three  are  apparently  of  a  magnitude  to  warrant  such 
designation.  These  are  the  Huronian  glaciation,  that  of 
the  *  Permo-Carbonif erous, '  and  that  of  the  Pleistocene. 
The  others,  so  far  as  available  data  go,  appear  to  be 
explainable  as  more  or  less  local  manifestations  that  had 
no  widespread  effect  on,  for  instance,  ocean  tempera- 
tures, distribution  of  life,  et  cetera.  They  might  well  have 
been  of  the  type  of  ordinary  mountain  glaciers,  due  en- 
tirely to  local  elevation  and  precipitation. "  .  .  .  ' '  If  the 
sun  had  been  the  principal  source  of  heat  in  pre-Pleisto- 
cene  time,  terrestrial  temperatures  would  of  necessity 
have  been  disposed  in  zones,  whereas  the  whole  trend  of 
this  paper  has  been  the  presentation  of  proof  that  these 
temperatures  were  distinctly  non-zonal.  Therefore  it 
seems  to  follow  that  the  sun — at  least  the  present  small- 
angle  sun — could  not  have  been  the  sole  or  even  the  prin- 
cipal source  of  heat  that  warmed  the  early  oceans. ' ' 

Knowlton  is  so  strongly  impressed  by  the  widespread 
fossil  floras  that  usually  occur  in  the  middle  parts  of  the 
geological  periods,  that  as  Schuchert3  puts  it,  he  neglects 
the  evidence  of  other  kinds.  In  the  middle  of  the  periods 
and  eras  the  expansion  of  the  warm  oceans  over  the  con- 
tinents was  greatest,  while  the  lands  were  small  and 
hence  had  more  or  less  insular  climates  of  the  oceanic 
type.  At  such  times,  the  marine  fauna  agrees  with  the 

3  Chas.  Schuchert :  Eeview  of  Knowlton 's  Evolution  of  Geological  Cli- 
mates, in  Am.  Jour.  ScL,  1921. 


CAUSES  OF  MILD  GEOLOGICAL  CLIMATES   169 

flora  in  indicating  a  mild  climate.  Large  colony-forming 
foraminifera,  stony  corals,  shelled  cephalopods,  gastro- 
pods and  thick-shelled  bivalves,  generally  the  cemented 
forms,  were  common  in  the  Far  North  and  even  in  the 
Arctic.  This  occurred  in  the  Silurian,  Devonian,  Penn- 
sylvanian,  and  Jurassic  periods,  yet  at  other  times,  such 
as  the  Cretaceous  and  Eocene,  such  forms  were  very 
greatly  reduced  in  variety  in  the  northern  regions  or  else 
wholly  absent.  These  things,  as  Schuchert3  says,  can  only 
mean  that  Knowlton  is  right  when  he  states  that  "cli- 
matic zoning  such  as  we  have  had  since  the  beginning  of 
the  Pleistocene  did  not  obtain  in  the  geologic  ages  prior 
to  the  Pleistocene."  It  does  not  mean,  however,  that 
there  was  a  "non-zonal  arrangement"  and  that  the  tem- 
perature of  the  oceans  was  everywhere  the  same  and 
"without  widespread  effect  on  the  distribution  of  life." 
Students  of  paleontology  hold  that  as  far  back  as  we 
can  go  in  the  study  of  plants,  there  are  evidences  of  sea- 
sons and  of  relatively  cool  climates  in  high  latitudes.  The 
cycads,  for  instance,  are  one  of  the  types  most  often  used 
as  evidence  of  a  warm  climate.  Yet  Wieland,4  who  has 
made  a  lifelong  study  of  these  plants,  says  that  many  of 
them  "might  well  grow  in  temperate  to  cool  climates. 
Until  far  more  is  learned  about  them  they  should  at  least 
be  held  as  valueless  as  indices  of  tropic  climates."  The 
inference  is  "that  either  they  or  their  close  relatives  had 
the  capacity  to  live  in  every  clime.  There  is  also  a  sus- 
picion that  study  of  the  associated  ferns  may  compel  re- 
vision of  the  long-accepted  view  of  the  universality  of 
tropic  climates  throughout  the  Mesozoic."  Nathorst  is 
quoted  by  Wieland  as  saying,  "I  think  .  .  .  that  during 
the  time  when  the  Gingkophytes  and  Cycadophytes  domi- 

*G.  E.  Wieland:  Distribution  and  Kelationships  of  the  Cycadeoids;  Am. 
Jour.  Bot,  Vol.  7,  1920,  pp.  125-145. 


170  CLIMATIC  CHANGES 

nated,  many  of  them  must  have  adapted  themselves  for 
living  in  cold  climates  also.  Of  this  I  have  not  the  least 
doubt." 

Another  important  line  of  evidence  which  Knowlton 
and  others  have  cited  as  a  proof  of  the  non-zonal  arrange- 
ment of  climate  in  the  past,  is  the  vast  red  beds  which  are 
found  in  the  Proterozoic,  late  Silurian,  Devonian,  Per- 
mian, and  Triassic,  and  in  some  Tertiary  formations. 
These  are  believed  to  resemble  laterite,  a  red  and  highly 
oxidized  soil  which  is  found  in  great  abundance  in  equa- 
torial regions.  Knowlton  does  not  attempt  to  show  that 
the  red  beds  present  equatorial  characteristics  in  other 
respects,  but  bases  his  conclusion  on  the  statement  that 
"red  beds  are  not  being  formed  at  the  present  time  in 
any  desert  region."  This  is  certainly  an  error.  As  has 
already  been  said,  in  both  the  Transcaspian  and  Takla 
Makan  deserts,  the  color  of  the  sand  regularly  changes 
from  brown  on  the  borders  to  pale  red  far  out  in  the 
desert.  Kuzzil  Kum,  or  Eed  Sand,  is  the  native  name. 
The  sands  in  the  center  of  the  desert  apparently  were 
originally  washed  down  from  the  same  mountains  as 
those  on  the  borders,  and  time  has  turned  them  red. 
Since  the  same  condition  is  reported  from  the  Arabian 
Desert,  it  seems  that  redness  is  characteristic  of  some  of 
the  world's  greatest  deserts.  Moreover,  beds  of  salt  and 
gypsum  are  regularly  found  in  red  beds,  and  they  can 
scarcely  originate  except  in  deserts,  or  in  shallow  almost 
landlocked  bays  on  the  coasts  of  deserts,  *as  appears  to 
have  happened  in  the  Silurian  where  marine  fossils  are 
found  interbedded  with  gypsum. 

Again,  Knowlton  says  that  red  beds  cannot  indicate 
deserts  because  the  plants  found  in  them  are  not 
"pinched  or  depauperate,  nor  do  they  indicate  xero- 
phytic  adaptations.  Moreover,  very  considerable  deposits 


CAUSES  OF  MILD  GEOLOGICAL  CLIMATES   171 

of  coal  are  found  in  red  beds  in  many  parts  of  the  world, 
which  implies  the  presence  of  swamps  but  little  above 
sea-level. " 

Students  of  desert  botany  are  likely  to  doubt  the  force 
of  these  considerations.  As  MacDougal5  has  shown,  the 
variety  of  plants  in  degertg  is  greater  than  in  moist 
regions.  Not  only  do  xerophytic  desert  species  prevail, 
but  halophytes  are  present  in  the  salty  areas,  and  hygro- 
phytes  in  the  wet  swampy  areas,  while  ordinary  meso- 
phytes  prevail  along  the  water  courses  and  are  washed 
down  from  the  mountains.  The  ordinary  plants,  not  the 
xerophytes,  are  the  ones  that  are  chiefly  preserved  since 
they  occur  in  most  abundance  near  streams  where  deposi- 
tion is  taking  place.  So  far  as  swamps  are  concerned,  few 
are  of  larger  size  than  those  of  Seistan  in  Persia,  Lop 
Nor  in  Chinese  Turkestan,  and  certain  others  in  the  midst 
of  the  Asiatic  deserts.  Streams  flowing  from  the  moun- 
tains into  deserts  are  almost  sure  to  form  large  swamps, 
such  as  those  along  the  Tarim  Eiver  in  central  Asia. 
Lake  Chad  in  Africa  is  another  example.  In  it,  too,  reeds 
are  very  numerous. 

Putting  together  the  evidence  on  both  sides  in  this  dis- 
puted question,  it  appears  that  throughout  most  of  geo- 
logical time  there  is  some  evidence  of  a  zonal  arrange- 
ment of  climate.  The  evidence  takes  the  form  of  traces  of 
cool  climates,  of  seasons,  and  of  deserts.  Nevertheless, 
there  is  also  strong  evidence  that  these  conditions  were 
in  general  less  intense  than  at  present  and  that  times  of 
relatively  warm,  moist  climate  without  great  seasonal 
extremes  have  prevailed  very  widely  during  periods 
much  longer  than  those  when  a  zonal  arrangement  as 

6D.  T.  MacDougal:  Botanical  Features  of  North  American  Deserts; 
Carnegie  Instit.  of  Wash.,  No.  99,  1908. 


172  CLIMATIC  CHANGES 

marked  as  that  of  today  prevailed.  As  Schuchert6  puts  it : 
* '  Today  the  variation  on  land  between  the  tropics  and  the 
poles  is  roughly  between  110°  and  — 60°F.,  in  the  oceans 
between  85°  and  31°F.  In  the  geologic  past  the  tempera- 
ture of  the  oceans  for  the  greater  parts  of  the  periods 
probably  was  most  often  between  85°  and  55 °F.,  while  on 
land  it  may  have  varied  between  90°  and  0°F.  At  rare 
intervals  the  extremes  were  undoubtedly  as  great  as  they 
are  today.  The  conclusion  is  therefore  that  at  all  times 
the  earth  had  temperature  zones,  varying  between  the 
present-day  intensity  and  times  which  were  almost  with- 
out such  belts,  and  at  these  latter  times  the  greater  part 
of  the  earth  had  an  almost  uniformly  mild  climate,  with- 
out winters. ' ' 

It  is  these  mild  climates  which  we  must  now  attempt 
to  explain.  This  leads  us  to  inquire  what  would  happen  to 
the  climate  of  the  earth  as  a  whole  if  the  conditions  which 
now  prevail  at  times  of  few  sunspots  were  to  become 
intensified.  That  they  could  become  greatly  intensified 
seems  highly  probable,  for  there  is  good  reason  to  think 
that  aside  from  the  sunspot  cycle  the  sun's  atmosphere 
is  in  a  disturbed  condition.  The  prominences  which 
sometimes  shoot  out  hundreds  of  thousands  of  miles 
seem  to  be  good  evidence  of  this.  Suppose  that  the  sun's 
atmosphere  should  become  very  quiet.  This  would  appar- 
ently mean  that  cyclonic  storms  would  be  much  less 
numerous  and  less  severe  than  during  the  present  times 
of  sunspot  minima.  The  storms  would  also  apparently 
follow  paths  in  middle  latitudes  somewhat  as  they  do 
now  when  sunspots  are  fewest.  The  first  effect  of  such  a 
condition,  if  we  can  judge  from  what  happens  at  present, 
would  be  a  rise  in  the  general  temperature  of  the  earth, 
because  less  heat  would  be  carried  aloft  by  storms. 

e  Loc.  cit. 


CAUSES  OF  MILD  GEOLOGICAL  CLIMATES    173 

Today,  as  is  shown  in  Earth  and  Sun,  a  difference  of 
perhaps  10  per  cent  in  the  average  storminess  during 
periods  of  sunspot  maxima  and  minima  is  correlated  with 
a  difference  of  3°C.  in  the  temperature  at  the  earth's 
surface.  This  includes  not  only  an  actual  lowering  of 
0.6° C.  at  times  of  sunspot  maxima,  but  the  overcoming 
of  the  effect  of  increased  insolation  at  such  times,  an 
effect  which  Abbot  calculates  as  about  2.5°C.  If  the 
storminess  were  to  be  reduced  to  one-half  or  one-quarter 
its  present  amount  at  sunspot  minima,  not  only  would  the 
loss  of  heat  by  upward  convection  in  storms  be  dimin- 
ished, but  the  area  covered  by  clouds  would  diminish  so 
that  the  sun  would  have  more  chance  to  warm  the  lower 
air.  Hence  the  average  rise  of  temperature  might  amount 
to  as  much  at  5°  or  10 °C. 

Another  effect  of  the  decrease  in  storminess  would  be 
to  make  the  so-called  westerly  winds,  which  are  chiefly 
southwesterly  in  the  northern  hemisphere  and  north- 
westerly in  the  southern  hemisphere,  more  strong  and 
steady  than  at  present.  They  would  not  continually  suffer 
interruption  by  cyclonic  winds  from  other  directions,  as 
is  now  the  case,  and  would  have  a  regularity  like  that 
of  the  trades.  This  conclusion  is  strongly  reenforced  in 
a  paper  by  Clayton7  which  came  to  hand  after  this  chap- 
ter had  been  completed.  From  his  studies  of  the  solar 
constant  and  the  temperature  of  the  earth  which  are 
described  in  Earth  and  Sun,  he  reaches  the  following 
conclusion:  "The  results  of  these  researches  have  led 
me  to  believe :  1.  That  if  there  were  no  variation  in  solar 
radiation  the  atmospheric  motions  would  establish  a 
stable  system  with  exchanges  of  air  between  equator  and 
pole  and  between  ocean  and  land,  in  which  the  only  varia- 

i  H.  H.  Clayton:  Variation  in  Solar  Eadiation  and  the  Weather;  Smiths. 
Misc.  Coll.,  Vol.  71,  No.  3,  Washington,  1920. 


174  CLIMATIC  CHANGES 

tions  would  be  daily  and  annual  changes  set  in  operation 
by  the  relative  motions  of  the  earth  and  sun.  2.  The  exist- 
ing abnormal  changes,  which  we  call  weather,  have  their 
origins  chiefly,  if  not  entirely,  in  the  variations  of  solar 
radiation. ' ' 

If  cyclonic  storms  and  " weather"  were  largely  elimi- 
nated and  if  the  planetary  system  of  winds  with  its 
steady  trades  and  southwesterlies  became  everywhere 
dominant,  the  regularity  and  volume  of  the  poleward- 
flowing  currents,  such  as  the  Gulf  Stream  and  the 
Atlantic  Drift  in  one  ocean,  and  the  Japanese  Current  in 
another,  would  be  greatly  increased.  How  important  this 
is  may  be  judged  from  the  work  of  Helland-Hansen  and 
Nansen.8  These  authors  find  that  with  the  passage  of  each 
cyclonic  storm  there  is  a  change  in  the  temperature  of 
the  surface  water  of  the  Atlantic  Ocean.  Winds  at  right 
angles  to  the  course  of  the  Drift  drive  the  water  first  in 
one  direction  and  then  in  the  other  but  do  not  advance  it 
in  its  course.  Winds  with  an  easterly  component,  on  the 
other  hand,  not  only  check  the  Drift  but  reverse  it,  driv- 
ing the  warm  water  back  toward  the  southwest  and 
allowing  cold  water  to  well  up  in  its  stead.  The  driving 
force  in  the  Atlantic  Drift  is  merely  the  excess  of  the 
winds  with  a  westerly  component  over  those  with  an 
easterly  component. 

Suppose  that  the  numbers  in  Fig.  8  represent  the 
strength  of  the  winds  in  a  certain  part  of  the  North 
Atlantic  or  North  Pacific,  that  is,  the  total  number  of 
miles  moved  by  the  air  per  year.  In  quadrant  A  of  the 
left-hand  part  all  the  winds  move  from  a  more  or  less 
southwesterly  direction  and  produce  a  total  movement 

8B.  Helland-Hansen  and  F.  Nansen:  Temperature  Variations  in  the 
North  Atlantic  Ocean  and  in  the  Atmosphere;  Misc.  Coll.,  Smiths.  Inst.,  Vol. 
70,  No.  4,  Washington,  1920. 


CAUSES  OF  MILD  GEOLOGICAL  CLIMATES    175 


20, 


C' 


25 


A' 


D1 


60 


Fig.  8.  Effect  of  diminution  of  storms  on 
movement  of  water. 

of  the  air  amounting  to  thirty  units  per  year.  Those 
coming  from  points  between  north  and  west  move  twenty- 
five  units;  those  between  north  and  east,  twenty  units; 
and  those  between  east  and  south,  twenty-five  units. 
Since  the  movement  of  the  winds  in  quadrants  B  and 
D  is  the  same,  these  winds  have  no  effect  in  producing 
currents.  They  merely  move  the  water  back  and  forth, 
and  thus  give  it  time  to  lose  whatever  heat  it  has  brought 
from  more  southerly  latitudes.  On  the  other  hand,  since 
the  easterly  winds  in  quadrant  C  do  not  wholly  check  the 
currents  caused  by  the  westerly  winds  of  quadrant  A, 
the  effective  force  of  the  westerly  winds  amounts  to  ten, 
or  the  difference  between  a  force  of  thirty  in  quadrant  A 
and  of  twenty  in  quadrant  C.  Hence  the  water  is  moved 
forward  toward  the  northeast,  as  shown  by  the  thick 
part  of  arrow  A. 

Now  suppose  that  cyclonic  storms  should  be  greatly 
reduced  in  number  so  that  in  the  zone  of  prevailing 
westerlies  they  were  scarcely  more  numerous  than  tropi- 


176  CLIMATIC  CHANGES 

cal  hurricanes  now  are  in  the  trade-wind  belt.  Then  the 
more  or  less  southwesterly  winds  in  quadrant  A'  in  the 
right-hand  part  of  Fig.  8  would  not  only  become  more 
frequent  but  would  be  stronger  than  at  present.  The 
total  movement  from  that  quarter  might  rise  to  sixty 
units,  as  indicated  in  the  figure.  In  quadrants  B'  and  D' 
the  movement  would  fall  to  fifteen  and  in  quadrant  C'  to 
ten.  B'  and  D'  would  balance  one  another  as  before.  The 
movement  in  A',  however,  would  exceed  that  in  C'  by  fifty 
instead  of  ten.  In  other  words,  the  current-making  force 
would  become  five  times  as  great  as  now.  The  actual 
effect  would  be  increased  still  more,  for  the  winds  from 
the  southwest  would  be  stronger  as  well  as  steadier  if 
there  were  no  storms.  A  strong  wind  which  causes  white- 
caps  has  much  more  power  to  drive  the  water  forward 
than  a  weaker  wind  which  does  not  cause  whitecaps.  In  a 
wave  without  a  whitecap  the  water  returns  to  practically 
the  original  point  after  completing  a  circle  beneath  the 
surface.  In  a  wave  with  a  whitecap,  however,  the  cap 
moves  forward.  Any  increase  in  velocity  beyond  the  rate 
at  which  whitecaps  are  formed  has  a  great  influence  upon 
the  amount  of  water  which  is  blown  forward.  Several 
times  as  much  water  is  drifted  forward  by  a  persistent 
wind  of  twenty  miles  an  hour  as  by  a  ten-mile  wind.9 

In  this  connection  a  suggestion  which  is  elaborated  in 
Chapter  XIII  may  be  mentioned.  At  present  the  salinity 
of  the  oceans  checks  the  general  deep-sea  circulation  and 
thereby  increases  the  contrasts  from  zone  to  zone.  In  the 
past,  however,  the  ocean  must  have  been  fresher  than 
now.  Hence  the  circulation  was  presumably  less  impeded, 
and  the  transfer  of  heat  from  low  latitudes  to  high  was 
facilitated. 

8  The  climatic  significance  of  ocean  currents  is  well  discussed  in  Croll's 
Climate  and  Time,  1875,  and  his  Climate  and  Cosmogony,  1889. 


CAUSES  OF  MILD  GEOLOGICAL  CLIMATES   177 

Consider  now  the  magnitude  of  the  probable  effect  of 
a  diminution  in  storms.  Today  off  the  coast  of  Norway 
in  latitude  65°N.  and  longitude  10°E.,  the  mean  tempera- 
ture in  January  is  2°C.  and  in  July  12° C.  This  represents 
a  plus  anomaly  of  about  22°  in  January  and  2°  in  July; 
that  is,  the  Norwegian  coast  is  warmer  than  the  normal 
for  its  latitude  by  these  amounts.  Suppose  that  in  some 
past  time  the  present  distribution  of  lands  and  seas  pre- 
vailed, but  Norway  was  a  lowland  where  extensive  de- 
posits could  accumulate  in  great  flood  plains.  Suppose, 
also,  that  the  sun's  atmosphere  was  so  inactive  that  few 
cyclonic  storms  occurred,  steady  winds  from  the  west- 
southwest  prevailed,  and  strong,  uninterrupted  ocean 
currents  brought  from  the  Caribbean  Sea  and  Gulf  of 
Mexico  much  greater  supplies  of  warm  water  than  at 
present.  The  Norwegian  winters  would  then  be  warmer 
than  now  not  only  because  of  the  general  increase  in  tem- 
perature which  the  earth  regularly  experiences  at  sun- 
spot  minima,  but  because  the  currents  would  accentuate 
this  condition.  In  summer  similar  conditions  would  pre- 
vail except  that  the  warming  effect  of  the  winds  and 
currents  would  presumably  be  less  than  in  winter,  but 
this  might  be  more  than  balanced  by  the  increased  heat 
of  the  sun  during  the  long  summer  days,  for  storms  and 
clouds  would  be  rare. 

If  such  conditions  raised  the  winter  temperature  only 
8°C.  and  the  summer  temperature  4°C.,  the  climate  would 
be  as  warm  as  that  of  the  northern  island  of  New  Zealand 
(latitude  35°-43°S.).  The  flora  of  that  part  of  New  Zea- 
land is  subtropical  and  includes  not  only  pines  and 
beeches,  but  palms  and  tree  ferns.  A  climate  scarcely 
warmer  than  that  of  New  Zealand  would  foster  a  flora 
like  that  which  existed  in  far  northern  latitudes  during 
some  of  the  milder  geological  periods.  If,  however,  the 


178  CLIMATIC  CHANGES 

general  temperature  of  the  earth's  surface  were  raised 
5°  because  of  the  scarcity  of  storms,  if  the  currents  were 
strong  enough  so  that  they  increased  the  present  anomaly 
by  50  per  cent,  and  if  more  persistent  sunshine  in  summer 
raised  the  temperature  at  that  season  about  4°C.,  the 
January  temperature  would  be  18 °C.  and  the  July  tem- 
perature 22 °C.  These  figures  perhaps  make  summer  and 
winter  more  nearly  alike  than  was  ever  really  the  case  in 
such  latitudes.  Nevertheless,  they  show  that  a  diminution 
of  storms  and  a  consequent  strengthening  and  steadying 
of  the  southwesterlies  might  easily  raise  the  temperature 
of  the  Norwegian  coast  so  high  that  corals  could  flourish 
within  the  Arctic  Circle. 

Another  factor  would  cooperate  in  producing  mild 
temperatures  in  high  latitudes  during  the  winter,  namely, 
the  fogs  which  would  presumably  accumulate.  It  is  well 
known  that  when  saturated  air  from  a  warm  ocean  is 
blown  over  the  lands  in  winter,  as  happens  so  often  in  the 
British  Islands  and  around  the  North  Sea,  fog  is  formed. 
The  effect  of  such  a  fog  is  indeed  to  shut  out  the  sun's 
radiation,  but  in  high  latitudes  during  the  winter  when 
the  sun  is  low,  this  is  of  little  importance.  Another  effect 
is  to  retain  the  heat  of  the  earth  itself.  When  a  constant 
supply  of  warm  water  is  being  brought  from  low  lati- 
tudes this  blanketing  of  the  heat  by  the  fog  becomes  of 
great  importance.  In  the  past,  whenever  cyclonic  storms 
were  weak  and  westerly  winds  were  correspondingly 
strong,  winter  fogs  in  high  latitudes  must  have  been  much 
more  widespread  and  persistent  than  now. 

The  bearing  of  fogs  on  vegetation  is  another  interest- 
ing point.  If  a  region  in  high  latitudes  is  constantly  pro- 
tected by  fog  in  winter,  it  can  support  types  of  vegetation 
characteristic  of  fairly  low  latitudes,  for  plants  are 
oftener  killed  by  dry  cold  than  by  moist  cold.  Indeed, 


CAUSES  OF  MILD  GEOLOGICAL  CLIMATES   179 

excessive  evaporation  from  the  plant  induced  by  dry 
cold  when  the  evaporated  water  cannot  be  rapidly  re- 
placed by  the  movement  of  sap  is  a  chief  reason  why 
large  plants  are  winterkilled.  The  growing  of  trans- 
planted palms  on  the  coast  of  southwestern  Ireland,  in 
spite  of  its  location  in  latitude  50°N.,  is  possible  only  be- 
cause of  the  great  fogginess  in  winter  due  to  the  marine 
climate.  The  fogs  prevent  the  escape  of  heat  and  ward  off 
killing  frosts.  The  tree  ferns  in  latitude  46°  S.  in  New 
Zealand,  already  referred  to,  are  often  similarly  pro- 
tected in  winter.  Therefore,  the  relative  frequency  of  fogs 
in  high  latitudes  when  storms  were  at  a  minimum  would 
apparently  tend  not  merely  to  produce  mild  winters  but 
to  promote  tropical  vegetation. 

The  strong  steady  trades  and  southwesterlies  which 
would  prevail  at  times  of  slight  solar  activity,  according 
to  our  hypothesis,  would  have  a  pronounced  effect  on  the 
water  of  the  deep  seas  as  well  as  upon  that  of  the  surface. 
In  the  first  place,  the  deep-sea  circulation  would  be  has- 
tened. For  convenience  let  us  speak  of  the  northern  hemi- 
sphere. In  the  past,  whenever  the  southwesterly  winds 
were  steadier  than  now,  as  was  probably  the  case  when 
cyclonic  storms  were  relatively  rare,  more  surface  water 
than  at  present  was  presumably  driven  from  low  latitudes 
and  carried  to  high  latitudes.  This,  of  course,  means  that 
a  greater  volume  of  water  had  to  flow  back  toward  the 
equator  in  the  lower  parts  of  the  ocean,  or  else  as  a  cool 
surface  current.  The  steady  southwesterly  winds,  how- 
ever, would  interfere  with  south-flowing  surface  currents, 
thus  compelling  the  polar  waters  to  find  their  way 
equatorward  beneath  the  surface.  In  low  latitudes  the 
polar  waters  would  rise  and  their  tendency  would  be  to 
lower  the  temperature.  Hence  steadier  westerlies  would 
make  for  lessened  latitudinal  contrasts  in  climate  not 


180  CLIMATIC  CHANGES 

only  by  driving  more  warm  water  poleward  but  by  caus- 
ing more  polar  water  to  reach  low  latitudes. 

At  this  point  a  second  important  consideration  must  be 
faced.  Not  only  would  the  deep-sea  circulation  be  has- 
tened, but  the  ocean  depths  might  be  warmed.  The  deep 
parts  of  the  ocean  are  today  cold  because  they  receive 
their  water  from  high  latitudes  where  it  sinks  because  of 
low  temperature.  Suppose,  however,  that  a  diminution  in 
storminess  combined  with  other  conditions  should  permit 
corals  to  grow  in  latitude  70°N.  The  ocean  temperature 
would  then  have  to  average  scarcely  lower  than  20°  C. 
and  even  in  the  coldest  month  the  water  could  scarcely 
fall  below  about  15°C.  Under  such  conditions,  if  the  polar 
ocean  were  freely  connected  with  the  rest  of  the  oceans, 
no  part  of  it  would  probably  have  a  temperature  much 
below  10°C.,  for  there  would  be  no  such  thing  as  ice  caps 
and  snowfields  to  reflect  the  scanty  sunlight  and  radiate 
into  space  what  little  heat  there  was.  On  the  contrary, 
during  the  winter  an  almost  constant  state  of  dense  fog- 
giness  would  prevail.  So  great  would  be  the  blanketing 
effect  of  this  that  a  minimum  monthly  temperature  of 
10°  C.  for  the  coldest  part  of  the  ocean  may  perhaps  be 
too  low  for  a  time  when  corals  thrived  in  latitude  70°. 

The  temperature  of  the  ocean  depths  cannot  perma- 
nently remain  lower  than  that  of  the  coldest  parts  of  the 
surface.  Temporarily  this  might  indeed  happen  when  a 
solar  change  first  reduced  the  storminess  and  strength- 
ened the  westerlies  and  the  surface  currents.  Gradually, 
however,  the  persistent  deep-sea  circulation  would  bring 
up  the  colder  water  in  low  latitudes  and  carry  downward 
the  water  of  medium  temperature  at  the  coldest  part  of 
the  surface.  Thus  in  time  the  whole  body  of  the  ocean 
would  become  warm.  The  heat  which  at  present  is  carried 
away 'from  the  earth's  surface  in  storms  would  slowly 


CAUSES  OF  MILD  GEOLOGICAL  CLIMATES   181 

accumulate  in  the  oceans.  As  the  process  went  on,  all 
parts  of  the  ocean's  surface  would  become  warmer,  for 
equatorial  latitudes  would  be  less  and  less  cooled  by  cold 
water  from  below,  while  the  water  blown  from  low  lati- 
tudes to  high  would  be  correspondingly  warmer.  The 
warming  of  the  ocean  would  come  to  an  end  only  with 
the  attainment  of  a  state  of  equilibrium  in  which  the  loss 
of  heat  by  radiation  and  evaporation  from  the  ocean's 
surface  equaled  the  loss  which  under  other  circumstances 
would  arise  from  the  rise  of  warm  air  in  cyclonic  storms. 
When  once  the  oceans  were  warmed,  they  would  form  an 
extremely  strong  conservative  force  tending  to  preserve 
an  equable  climate  in  all  latitudes  and  at  all  seasons. 
According  to  the  solar  cyclonic  hypothesis  such  condi- 
tions ought  to  have  prevailed  throughout  most  of  geo- 
logical time.  Only  after  a  strong  and  prolonged  solar 
disturbance  with  its  consequent  storminess  would  condi- 
tions like  those  of  today  be  expected. 

In  this  connection  another  possibility  may  be  men- 
tioned. It  is  commonly  assumed  that  the  earth's  axis  is 
held  steadily  in  one  direction  by  the  fact  that  the  rotating 
earth  is  a  great  gyroscope.  Having  been  tilted  to  a  cer- 
tain position,  perhaps  by  some  extraneous  force,  the  axis 
is  supposed  to  maintain  that  position  until  some  other 
force  intervenes.  Cordeiro,10  however,  maintains  that  this 
is  true  only  of  an  absolutely  rigid  gyroscope.  He  believes 
that  it  is  mathematically  demonstrable  that  if  an  elastic 
gyroscope  be  gradually  tilted  by  some  extraneous  force, 
and  if  that  force  then  ceases  to  act,  the  gyroscope  as  a 
whole  will  oscillate  back  and  forth.  The  earth  appears  to 
be  slightly  elastic.  Cordeiro  therefore  applies  his  for- 
mulae to  it,  on  the  following  assumptions:  (1)  That  the 
original  position  of  the  axis  was  nearly  vertical  to  the 

10  F.  J.  B.  Cordeiro :  The  Gyroscope,  1913. 


182  CLIMATIC  CHANGES 

plane  of  the  ecliptic  in  which  the  earth  revolves  around 
the  sun;  (2)  that  at  certain  times  the  inclination  has  been 
even  greater  than  now;  and  (3)  that  the  position  of  the 
axis  with  reference  to  the  earth  has  not  changed  to  any 
great  extent,  that  is,  the  earth's  poles  have  remained 
essentially  stationary  with  reference  to  the  earth,  al- 
though the  whole  earth  has  been  gyroscopically  tilted 
back  and  forth  repeatedly. 

With  a  vertical  axis  the  daylight  and  darkness  in  all 
parts  of  the  earth  would  be  of  equal  duration,  being 
always  twelve  hours.  There  would  be  no  seasons,  and  the 
climate  would  approach  the  average  condition  now  ex- 
perienced at  the  two  equinoxes.  On  the  whole  the  climate 
of  high  latitudes  would  give  the  impression  of  being 
milder  than  now,  for  there  would  be  less  opportunity  for 
the  accumulation  of  snow  and  ice  with  their  strong  cool- 
ing effect.  On  the  other  hand,  if  the  axis  were  tilted  more 
than  now,  the  winter  nights  would  be  longer  and  the 
winters  more  severe  than  at  present,  and  there  would  be 
a  tendency  toward  glaciation.  Thus  Cordeiro  accounts 
for  alternating  mild  and  glacial  epochs.  The  entire  swing 
from  the  vertical  position  to  the  maximum  inclination 
and  back  to  the  vertical  may  last  millions  of  years  de- 
pending on  the  earth's  degree  of  elasticity.  The  swing 
beyond  the  vertical  position  in  the  other  direction  would 
be  equally  prolonged.  Since  the  axis  is  now  supposed  to 
be  much  nearer  its  maximum  than  its  minimum  degree  of 
tilting,  the  duration  of  epochs  having  a  climate  more 
severe  than  that  of  the  present  would  be  relatively  short, 
while  the  mild  epochs  would  be  long. 

Cordeiro 's  hypothesis  has  been  almost  completely 
ignored.  One  reason  is  that  his  treatment  of  geological 
facts,  and  especially  his  method  of  riding  rough-shod 
over  widely  accepted  conclusions,  has  not  commended  his 


CAUSES  OF  MILD  GEOLOGICAL  CLIMATES   183 

work  to  geologists.  Therefore  they  have  not  deemed  it 
worth  while  to  urge  mathematicians  to  test  the  assump- 
tions and  methods  by  which  he  reached  his  results.  It  is 
perhaps  unfair. to  test  Cordeiro  by  geology,  for  he  lays 
no  claim  to  being  a  geologist.  In  mathematics  he  labors 
under  the  disadvantage  of  having  worked  outside  the 
usual  professional  channels,  so  that  his  work  does  not 
seem  to  have  been  subjected  to  sufficiently  critical 
analysis. 

Without  expressing  any  opinion  as  to  the  value  of 
Cordeiro 's  results  we  feel  that  the  subject  of  the  earth's 
gyroscopic  motion  and  of  a  possible  secular  change  in 
the  direction  of  the  axis  deserves  investigation  for  two 
chief  reasons.  In  the  first  place,  evidences  of  seasonal 
changes  and  of  seasonal  uniformity  seem  to  occur  more 
or  less  alternately  in  the  geological  record.  Second,  the 
remarkable  discoveries  of  Garner  and  Allard11  show  that 
the  duration  of  daylight  has  a  pronounced  effect  upon 
the  reproduction  of  plants.  We  have  referred  repeatedly 
to  the  tree  ferns,  corals,  and  other  forms  of  life  which 
now  live  in  relatively  low  latitudes  and  which  cannot 
endure  strong  seasonal  contrasts,  but  which  once  lived 
far  to  the  north.  On  the  other  hand,  Sayles,12  for  example, 
finds  that  microscopical  examination  of  the  banding  of 
ancient  shales  and  slates  indicates  distinct  seasonal  band- 
ing like  that  of  recent  Pleistocene  clays  or  of  the  Squan- 
tum  slate  formed  during  or  near  the  Permian  glacial 
period.  Such  seasonal  banding  is  found  in  rocks  of  vari- 
ous ages:  (a)  Huronian,  in  cobalt  shales  previously 
reported  by  Coleman;  (b)  late  Proterozoic  or  early  Cam- 

11  W.  W.  Garner  and  H.  A.  Allard:  Flowering  and  Fruition  of  Plants 
as  Controlled  by  Length  of  Day;  Yearbook  Dept.  Agri.,  1920,  pp.  377-400. 

12  Eeport  of   Committee  on   Sedimentation,  National   Kesearch  Council, 
April,  1922. 


184  CLIMATIC  CHANGES 

brian,  in  Hiwassee  slate;  (c)  lower  Cambrian,  in  Geor- 
gian slates  of  Vermont ;  (d)  lower  Ordovician,  in  Georgia 
(Eockmart  slate),  Tennessee  (Athens  shale),  Vermont 
(slates),  and  Quebec  (Beekmantown  formation) ;  and  (e) 
Permian  in  Massachusetts  (Squantum  slate).  How  far 
the  periods  during  which  such  evidence  of  seasons  was 
recorded  really  alternated  with  mild  periods,  when  tropi- 
cal species  lived  in  high  latitudes  and  the  contrast  of 
seasons  was  almost  or  wholly  lacking,  we  have  as  yet  no 
means  of  knowing.  If  periods  characterized  by  marked 
seasonal  changes  should  be  found  to  have  alternated  with 
those  when  the  seasons  were  of  little  importance,  the  fact 
would  be  of  great  geological  significance. 

The  discoveries  of  Garner  and  Allard  as  to  the  effect 
of  light  on  reproduction  began  with  a  peculiar  tobacco 
plant  which  appeared  in  some  experiments  at  Washing- 
ton. The  plant  grew  to  unusual  size,  and  seemed  to 
promise  a  valuable  new  variety.  It  formed  no  seeds,  how- 
ever, before  the  approach  of  cold  weather.  It  was  there- 
fore removed  to  a  greenhouse  where  it  flowered  and 
produced  seed.  In  succeeding  years  the  flowering  was 
likewise  delayed  till  early  winter,  but  finally  it  was  dis- 
covered that  if  small  plants  were  started  in  the  green- 
house in  the  early  fall  they  flowered  at  the  same  time  as 
the  large  ones.  Experiments  soon  demonstrated  that  the 
time  of  flowering  depends  largely  upon  the  length  of  the 
daily  period  when  the  plants  are  exposed  to  light.  The 
same  is  true  of  many  other  plants,  and  there  is  great 
variety  in  the  conditions  which  lead  to  flowering.  Some 
plants,  such  as  witch  hazel,  appear  to  be  stimulated  to 
bloom  by  very  short  days,  while  others,  such  as  evening 
primrose,  appear  to  require  relatively  long  days.  So 
sensitive  are  plants  in  this  respect  that  Garner  and 
Allard,  by  changing  the  length  of  the  period  of  light,  have 


CAUSES  OF  MILD  GEOLOGICAL  CLIMATES    185 

caused  a  flowerbud  in  its  early  stages  not  only  to  stop 
developing  but  to  return  once  more  to  a  vegetative  shoot. 

Common  iris,  which  flowers  in  May  and  June,  will  not  blossom 
under  ordinary  conditions  when  grown  in  the  greenhouse  in 
winter,  even  under  the  same  temperature  conditions  that  prevail 
in  early  summer.  Again,  one  variety  of  soy  beans  will  regularly 
begin  to  flower  in  June  of  each  year,  a  second  variety  in  July, 
and  a  third  in  August,  when  all  are  planted  on  the  same  date. 
There  are  no  temperature  differences  during  the  summer  months 
which  could  explain  these  differences  in  time  of  flowering;  and, 
since  "internal  causes"  alone  cannot  be  accepted  as  furnishing 
a  satisfactory  explanation,  some  external  factor  other  than  tem- 
perature must  be  responsible. 

The  ordinary  varieties  of  cosmos  regularly  flower  in  the  fall 
in  northern  latitudes  if  they  are  planted  in  the  spring  or  summer. 
If  grown  in  a  warm  greenhouse  during  the  winter  months  the 
plants  also  flower  readily,  so  that  the  cooler  weather  of  fall  is 
not  a  necessary  condition.  If  successive  plantings  of  cosmos  are 
made  in  the  greenhouse  during  the  late  winter  and  early  spring 
months,  maintaining  a  uniform  temperature  throughout,  the 
plantings  made  after  a  certain  date  will  fail  to  blossom  promptly, 
but,  on  the  contrary,  will  continue  to  grow  till  the  following  fall, 
thus  flowering  at  the  usual  season  for  this  species.  This  curious 
reversal  of  behavior  with  advance  of  the  season  cannot  be  attrib- 
uted to  change  in  temperature.  Some  other  factor  is  responsible 
for  the  failure  of  cosmos  to  blossom  during  the  summer  months. 
In  this  respect  the  behavior  of  cosmos  is  just  the  opposite  of  that 
observed  in  iris. 

Certain  varieties  of  soy  beans  change  their  behavior  in  a 
peculiar  manner  with  advance  of  the  Summer  season.  The  variety 
known  as  Biloxi,  for  example,  when  planted  early  in  the  spring 
in  the  latitude  of  Washington,  D.  C.,  continues  to  grow  through- 
out the  summer,  flowering  in  September.  The  plants  maintain 
growth  without  flowering  for  fifteen  to  eighteen  weeks,  attaining 
a  height  of  five  feet  or  more.  As  the  dates  of  successive  plantings 
are  moved  forward  through  the  months  of  June  and  July,  how- 


186  CLIMATIC  CHANGES 

ever,  there  is  a  marked  tendency  for  the  plants  to  cut  short  the 
period  of  growth  which  precedes  flowering.  This  means,  of  course, 
that  there  is  a  tendency  to  flower  at  approximately  the  same  time 
of  year  regardless  of  the  date  of  planting.  As  a  necessary  con- 
sequence, the  size  of  the  plants  at  the  time  of  flowering  is  reduced 
in  proportion  to  the  delay  in  planting. 

The  bearing  of  this  on  geological  problems  lies  in  a 
query  which  it  raises  as  to  the  ability  of  a  genus  or  family 
of  plants  to  adapt  itself  to  days  of  very  different  length 
from  those  to  which  it  is  wonted.  Could  tree  ferns,  gink- 
gos,  cycads,  and  other  plants  whose  usual  range  of  loca- 
tion never  subjects  them  to  daylight  for  more  than 
perhaps  fourteen  hours  or  less  than  ten,  thrive  and  re- 
produce themselves  if  subjected  to  periods  of  daylight 
ranging  all  the  way  from  nothing  up  to  about  twenty- 
four  hours'?  No  answer  to  this  is  yet  possible,  but  the 
question  raises  most  interesting  opportunities  of  in- 
vestigation. If  Cordeiro  is  right  as  to  the  earth's  elastic 
gyroscopic  motion,  there  may  have  been  certain  periods 
when  a  vertical  or  almost  vertical  axis  permitted  the 
days  to  be  of  almost  equal  length  at  all  seasons  in  all 
latitudes.  If  such  an  absence  of  seasons  occurred  when 
the  lands  were  low,  when  the  oceans  were  extensive  and 
widely  open  toward  the  poles,  and  when  storms  were 
relatively  inactive,  the  result  might  be  great  mildness  of 
climate  such  as  appears  sometimes  to  have  prevailed  in 
the  middle  of  geological  eras.  Suppose  on  the  other  hand 
that  the  axis  should  be  tilted  more  than  now,  and  that 
the  lands  should  be  widely  emergent  and  the  storm  belt 
highly  active  in  low  latitudes,  perhaps  because  of  the 
activity  of  the  sun.  The  conditions  might  be  favorable  for 
glaciation  at  latitudes  as  low  as  those  where  the  Permo- 
Carboniferous  ice  sheets  appear  to  have  centered.  The 
possibilities  thus  suggested  by  Cordeiro 's  hypothesis  are 


CAUSES  OF  MILD  GEOLOGICAL  CLIMATES   187 

so  interesting  that  the  gyroscopic  motion  of  the  earth 
ought  to  be  investigated  more  thoroughly.  Even  if  no 
such  gyroscopic  motion  takes  place,  however,  the  other 
causes  of  mild  climate  discussed  in  this  chapter  may 
be  enough  to  explain  all  the  observed  phenomena. 

Many  important  biological  consequences  might  be 
drawn  from  this  study  of  mild  geological  climates,  but 
this  book  is  not  the  place  for  them.  In  the  first  chapter 
we  saw  that  one  of  the  most  remarkable  features  of  the 
climate  of  the  earth  is  its  wonderful  uniformity  through 
hundreds  of  millions  of  years.  As  we  come  down  through 
the  vista  of  years  the  mild  geological  periods  appear  to 
represent  a  return  as  nearly  as  possible  to  this  standard 
condition  of  uniformity.  Certain  changes  of  the  earth 
itself,  as  we  shall  see  in  the  next  chapter,  may  in  the  long 
run  tend  slightly  to  change  the  exact  conditions  of  this 
climatic  standard,  as  we  might  perhaps  call  it.  Yet  they 
act  so  slowly  that  their  effect  during  hundreds  of  millions 
of  years  is  still  open  to  question.  At  most  they  seem 
merely  to  have  produced  a  slight  increase  in  diversity 
from  season  to  season  and  from  zone  to  zone.  The  normal 
climate  appears  still  toi  be  of  a  milder  type  than  that 
which  happens  to  prevail  at  present.  Some  solar  condi- 
tion, whose  possible  nature  will  be  discussed  later,  seems 
even  now  to  cause  the  number  of  cyclonic  storms  to  be 
greater  than  normal.  Hence  the  earth's  climate  still 
shows  something  of  the  great  diversity  of  seasons  and 
of  zones  which  is  so  marked  a  characteristic  of  glacial 
epochs. 


CHAPTER  XI 
TERRESTRIAL  CAUSES  OF  CLIMATIC  CHANGES 

THE  major  portion  of  this  book  has  been  concerned 
with  the  explanation  of  the  more  abrupt  and  ex- 
treme changes  of  climate.  This  chapter  and  the 
next  consider  two  other  sorts  of  climatic  changes,  the 
slight  secular  progression  during  the  hundreds  of  mil- 
lions of  years  of  recorded  earth  history,  and  especially 
the  long  slow  geologic  oscillations  of  millions  or  tens  of 
millions  of  years.  It  is  generally  agreed  among  geologists 
that  the  progressive  change  has  tended  toward  greater 
extremes  of  climate ;  that  is,  greater  seasonal  contrasts, 
and  greater  contrasts  from  place  to  place  and  from  zone 
to  zone.1  The  slow  cyclic  changes  have  been  those  that 
favored  widespread  glaciation  at  one  extreme  near  the 
ends  of  geologic  periods  and  eras,  and  mild  temperatures 
even  in  subpolar  regions  at  the  other  extreme  during  the 
medial  portions  of  the  periods. 

As  has  been  pointed  out  in  an  earlier  chapter,  it  has 
often  been  assumed  that  all  climatic  changes  are  due  to 
terrestrial  causes.  We  have  seen,  however,  that  there  is 
strong  evidence  that  solar  variations  play  a  large  part  in 
modifying  the  earth's  climate.  We  have  also  seen  that  no 
known  terrestrial  agency  appears  to  be  able  to  produce 
the  abrupt  changes  noted  in  recent  years,  the  longer 

iChas.  Schuchert:  The  Earth's  Changing  Surface  and  Climate  during 
Geologic  Time;  in  Lull:  The  Evolution  of  the  Earth  and  Its  Inhabitants, 
1918,  p.  55. 


TERRESTRIAL  CAUSES  OF  CHANGES         189 

cycles  of  historical  times,  or  geological  changes  of  the 
shorter  type,  such  as  glaciation.  Nevertheless,  terrestrial 
changes  doubtless  have  assisted  in  producing  both  the 
progressive  change  and  the  slow  cyclic  changes  recorded 
in  the  rocks,  and  it  is  the  purpose  of  this  chapter  and  the 
two  that  follow  to  consider  what  terrestrial  changes  have 
taken  place  and  the  probable  effect  of  such  changes. 

The  terrestrial  changes  that  have  a  climatic  signifi- 
cance are  numerous.  Some,  such  as  variations  in  the 
amount  of  volcanic  dust  in  the  higher  air,  have  been  con- 
sidered in  an  earlier  chapter.  Others  are  too  imperfectly 
known  to  warrant  discussion,  and  in  addition  there  are 
presumably  others  which  are  entirely  unknown.  Doubt- 
less some  of  these  little  known  or  unknown  changes  have 
been  of  importance  in  modifying  climate.  For  example, 
the  climatic  influence  of  vegetation,  animals,  and  man 
may  be  appreciable.  Here,  however,  we  shall  confine  our- 
selves to  purely  physical  causes,  which  will  be  treated  in 
the  following  order :  First,  those  concerned  with  the  solid 
parts  of  the  earth,  namely:  (I)  amount  of  land;  (II)  dis- 
tribution of  land;  (III)  height  of  land;  (IV)  lava  flows; 
and  (V)  internal  heat.  Second,  those  which  arise  from 
the  salinity  of  oceans,  and  third,  those  depending  on  the 
composition  and  amount  of  atmosphere. 

The  terrestrial  change  which  appears  indirectly  to 
have  caused  the  greatest  change  in  climate  is  the  con- 
traction of  the  earth.  The  problem  of  contraction  is 
highly  complex  and  is  as  yet  only  imperfectly  understood. 
Since  only  its  results  and  not  its  processes  influence  cli- 
mate, the  following  section  as  far  as  page  196  is  not 
necessary  to  the  general  reader.  It  is  inserted  in  order  to 
explain  why  we  assume  that  there  have  been  oscillations 
between  certain  types  of  distribution  of  the  lands. 

The  extent  of  the  earth's  contraction  may  be  judged 


190  CLIMATIC  CHANGES 

from  the  shrinkage  indicated  by  the  shortening  of  the 
rock  formations  in  folded  mountains  such  as  the  Alps, 
Juras,  Appalachians,  and  Caucasus.  Geologists  are  con- 
tinually discovering  new  evidence  of  thrust  faults  of 
great  magnitude  where  masses  of  rock  are  thrust  bodily 
over  other  rocks,  sometimes  for  many  miles.  Therefore, 
the  estimates  of  the  amount  of  shrinkage  based  on  the 
measurements  of  folds  and  faults  need  constant  revision 
upward.  Nevertheless,  they  have  already  reached  a  con- 
siderable figure.  For  example,  in  1919,  Professor  A.  Heim 
estimated  the  shortening  of  the  meridian  passing  through 
the  modern  Alps  and  the  ancient  Hercynian  and  Cale- 
donian mountains  as  fully  a  thousand  miles  in  Europe, 
and  over  five  hundred  miles  for  the  rest  of  this  meridian.2 
This  is  a  radial  shortening  of  about  250  miles.  Possibly 
the  shrinkage  has  been  even  greater  than  this.  Chamber- 
lin3  has  compared  the  density  of  the  earth,  moon,  Mars, 
and  Venus  with  one  another,  and  found  it  probable  that 
the  radial  shrinkage  of  the  earth  may  be  as  much  as 
570  miles.  This  result  is  not  so  different  from  Heim's  as 
appears  at  first  sight,  for  Heim  made  no  allowance  for 
unrecognized  thrust  faults  and  for  the  contraction  inci- 
dent to  metamorphism.  Moreover,  Heim  did  not  include 
shrinkage  during  the  first  half  of  geological  time  before 
the  above-mentioned  mountain  systems  were  upheaved. 
According  to  a  well-established  law  of  physics,  con- 
traction of  a  rotating  body  results  in  more  rapid  rotation 
and  greater  centrifugal  force.  These  conditions  must  in- 
crease the  earth's  equatorial  bulge  and  thereby  cause 
changes  in  the  distribution  of  land  and  water.  Opposed 
to  the  rearrangement  of  the  land  due  to  increased  rota- 

2  Quoted  by  J.  Cornet :  Cours  de  Geologic,  1920,  p.  330. 
a  T.  C.  Chamberlin :   The  Order  of  Magnitude  of  the  Shrinkage  of  the 
Earth;  Jour.  Geol.,  Vol.  28,  1920,  pp.  1-17,  126-157. 


TERRESTRIAL  CAUSES  OF  CHANGES         191 

tion  caused  by  contraction,  there  has  presumably  been 
another  rearrangement  due  to  tidal  retardation  of  the 
earth's  rotation  and  a  consequent  lessening  of  the  equa- 
torial bulge.  G.  H.  Darwin  long  ago  deduced  a  relatively 
large  retardation  due  to  lunar  tides.  A  few  years  ago 
W.  D.  MacMillan,  on  other  assumptions,  deduced  only  a 
negligible  retardation.  Still  more  recently  Taylor*  has 
studied  the  tides  of  the  Irish  Sea,  and  his  work  has  led 
Jeffreys5  and  Brown6  to  conclude  that  there  has  been  con- 
siderable retardation,  perhaps  enough,  according  to 
Brown,  to  equal  the  acceleration  due  to  the  earth's  con- 
traction. From  a  prolonged  and  exhaustive  study  of  the 
motions  of  the  moon  Brown  concludes  that  tidal  friction 
or  some  other  cause  is  now  lengthening  the  day  at  the  rate 
of  one  second  per  thousand  years,  or  an  hour  in  almost 
four  million  years  if  the  present  rate  continues.  He  makes 
it  clear  that  the  retardation  due  to  tides  would  not  corre-  ^2 
spond  in  point  of  time  with  the  acceleration  due  to  con-  Q  ~ 
traction.  The  retardation  would  occur  slowly,  and  would 
take  place  chiefly  during  the  long  quiet  periods  of  geo- 
logic history,  while  the  acceleration  would  occur  rapidly 
at  times  of  diastrophic  deformation.  As  a  consequence, 
the  equatorial  bulge  would  alternately  be  reduced  at  a 
slow  rate,  and  then  somewhat  suddenly  augmented. 

The  less  rigid  any  part  of  the  earth  is,  the  more  quickly 
it  responds  to  the  forces  which  lead  to  bulging  or  which 
tend  to  lessen  the  bulge.  Since  water  is  more  fluid  than 
land,  the  contraction  of  the  earth  and  the  tidal  retarda- 
tion presumably  tend  alternately  to  increase  and  decrease 
the  amount  of  water  near  the  equator  more  than  the 

*  G.  I.  Taylor :  Philosophical  Transactions,  A.  220,  1919,  pp.  1-33 ; 
Monthly  Notices  Eoyal  Astron.  Soc.,  Jan.,  1920,  Vol.  80,  p.  308. 

5J.  Jeffreys:  Monthly  Notices  Eoyal  Astron.  Soc.,  Jan.,  1920,  Vol.  80, 
p.  309. 

«  E.  W.  Brown :  personal  communication. 


192  CLIMATIC  CHANGES 

amount  of  land.  Thus,  throughout  geological  history  we 
should  look  for  cyclic  changes  in  the  relative  area  of  the 
lands  within  the  tropics  and  similar  changes  of  opposite 
phase  in  higher  latitudes.  The  extent  of  the  change  would 
depend  upon  (a)  the  amount  of  alteration  in  the  speed 
of  rotation,  and  (b)  the  extent  of  low  land  in  low  lati- 
tudes and  of  shallow  sea  in  high  latitudes.  According  to 
Slichter's  tables,  if  the  earth  should  rotate  in  twenty- 
three  hours  instead  of  twenty-four,  the  great  Amazon 
lowland  would  be  submerged  by  the  inflow  of  oceanic 
water,  while  wide  areas  in  Hudson  Bay,  the  North  Sea, 
and  other  northern  regions,  would  become  land  because 
the  ocean  water  would  flow  away  from  them.7 

Following  the  prompt  equatorward  movement  of  water 
which  would  occur  as  the  speed  of  rotation  increased, 
there  must  also  be  a  gradual  movement  or  creepage  of  the 
solid  rocks  toward  the  equator,  that  is,  a  bulging  of  the 
ocean  floor  and  of  the  lands  in  low  latitudes,  with  a  con- 
sequent emergence  of  the  lands  there  and  a  relative 
rise  of  sea  level  in  higher  latitudes.  Tidal  retardation 
would  have  a  similar  effect.  Suess8  has  described  wide- 
spread elevated  strand  lines  in  the  tropics  which  he  in- 
terprets as  indicating  a  relatively  sudden  change  in  sea 
level,  though  he  does  not  suggest  a  cause  of  the  change. 
However,  in  speaking  of  recent  geological  times,  Suess 
reports  that  a  movement  more  recent  than  the  old 
strands  "was  an  accumulation  of  water  toward  the 
equator,  a  diminution  toward  the  poles,  and  (it  appears) 
as  though  this  last  movement  were  only  one  of  the  many 
oscillations  which  succeed  each  other  with  the  same  tend- 
ency, i.e.,  with  a  positive  excess  at  the  equator,  a  nega- 

'  C.  S.  Slichter:  The  Eotational  Period  of  a  Heterogeneous  Spheroid;  in 
Contributions  to  the  Fundamental  Problems  of  Geology,  by  T.  C.  Cham- 
berlin,  et  al,  Carnegie  Inst.  of  Wash.,  No.  107,  1909. 

8E.  Suess:  The  Face  of  the  Earth,  Vol.  II,  p.  553,  1901. 


TERRESTRIAL  CAUSES  OF  CHANGES         193 

tive  excess  at  the  poles."  (Vol.  II,  p.  551.)  This  creepage 
of  the  rocks  equatorward  seemingly  might  favor  the 
growth  of  mountains  in  tropical  and  subtropical  regions, 
because  it  is  highly  improbable  that  the  increase  in  the 
bulge  would  go  on  in  all  longitudes  with  perfect  uni- 
formity. Where  it  went  on  most  rapidly  mountains  would 
arise.  That  such  irregularity  of  movement  has  actually 
occurred  is  suggested  not  only  by  the  fact  that  many 
Cenozoic  and  older  mountain  ranges  extend  east  and 
west,  but  by  the  further  fact  that  these  include  some  of 
our  greatest  ranges,  many  of  which  are  in  fairly  low  lati- 
tudes. The  Himalayas,  the  Javanese  ranges,  and  the  half- 
submerged  Caribbean  chains  are  examples.  Such  moun- 
tains suggest  a  thrust  in  a  north  and  south  direction 
which  is  just  what  would  happen  if  the  solid  mass  of  the 
earth  were  creeping  first  equatorward  and  then  poleward. 

A  fact  which  is  in  accord  with  the  idea  of  a  periodic 
increase  in  the  oceans  in  low  latitudes  because  of  renewed 
bulging  at  the  equator  is  the  exposure  in  moderately 
high  latitudes  of  the  greatest  extent  of  ancient  rocks. 
This  seems  to  mean  that  in  low  latitudes  the  frequent 
deepening  of  the  oceans  has  caused  the  old  rocks  to  be 
largely  covered  by  sediments,  while  the  old  lands  in 
higher  latitudes  have  been  left  more  fully  exposed  to 
erosion. 

Another  suggestion  of  such  periodic  equatorward  move- 
ments of  the  ocean  water  is  found  in  the  reported  contrast 
between  the  relative  stability  with  which  the  northern  part 
of  North  America  has  remained  slightly  above  sea  level 
except  at  times  of  widespread  submergence,  while  the 
southern  parts  have  suffered  repeated  submergence  al- 
ternating with  great  emergence.9  Furthermore,  although 

»  Chas.  Schuchert:  The  Earth's  Changing  Surface  and  Climate]  in  Lull: 
The  Evolution  of  the  Earth  and  Its  Inhabitants,  1918,  p.  78. 


194  CLIMATIC  CHANGES 

the  northern  part  of  North  America  has  been  generally 
exposed  to  erosion  since  the  Proterozoic,  it  has  supplied 
much  less  sediment  than  have  the  more  southern  land 
areas.10  This  apparently  means  that  much  of  Canada  has 
stood  relatively  low,  while  repeated  and  profound  uplift 
alternating  with  depression  has  occurred  in  subtropical 
latitudes,  apparently  in  adjustment  to  changes  in  the 
earth's  speed  of  rotation.  The  uplifts  generally  followed 
the  times  of  submergence  due  to  equatorward  movement 
of  the  water,  though  the  buckling  of  the  crust  which  ac- 
companies shrinkage  doubtless  caused  some  of  the  sub- 
mergence. The  evidence  that  northern  North  America 
stood  relatively  low  throughout  much  of  geological  time 
depends  not  only  on  the  fact  that  little  sediment  came  to 
the  south  from  the  north,  but  also  on  the  fact  that  at 
times  of  especially  widespread  epicontinental  seas,  the 
submergence  was  initiated  at  the  north.11  This  is  espe- 
cially true  for  Ordovician,  Silurian,  Devonian,  and  Juras- 
sic times  in  North  America.  General  submergence  of  this 
kind  is  supposed  to  be  due  chiefly  to  the  overflowing  of 
the  ocean  when  its  level  is  slowly  raised  by  the  deposition 
of  sediment  derived  from  the  erosion  of  what  once  were 
continental  highlands  but  later  are  peneplains.  The  fact 
that  such  submergence  began  in  high  latitudes,  however, 
seems  to  need  a  further  explanation.  The  bulging  of  the 
rock  sphere  at  the  equator  and  the  consequent  displace- 
ment of  some  of  the  water  in  low  latitudes  would  furnish 
such  an  explanation,  as  would  also  a  decrease  in  the  speed 
of  rotation  induced  by  tidal  retardation,  if  that  retarda- 
tion were  great  enough  and  rapid  enough  to  be  geologi- 
cally effective. 

10  J.  Barrell :   Khythms  and  the  Measurement  of  Geologic  Time;   Bull. 
Geol.  Soc.  Am.,  Vol.  28,  1917,  p.  838. 

11  Chas.  Schuchert :  loc.  cit.,  p.  78. 


TERRESTRIAL  CAUSES  OF  CHANGES         195 

The  climatic  effects  of  the  earth's  contraction,  which 
we  shall  shortly  discuss,  are  greatly  complicated  by  the 
fact  that  contraction  has  taken  place  irregularly.  Such 
irregularity  has  occurred  in  spite  of  the  fact  that  the 
processes  which  cause  contraction  have  probably  gone  (A^t^.\ 
on  quite  steadily  throughout  geological  history.  These   *flj£l 
processes  include  the  chemical  reorganization  of  the  mi" 
erals  of  the  crust,  a  process  which  is  illustrated  by  tL_ 
metamorphism    of   sedimentary   rocks    into   crystalline 
forms.  The  escape  of  gases  through  volcanic  action  or       {'' 
otherwise  has  been  another  important  process. 

Although  the  processes  which  cause  contraction  prob- 
ably go  on  steadily,  their  effect,  as  Chamberlin12  and  Q&C^j- 
others  have  pointed  out,  is  probably  delayed  by  inertia. 
Thus  the  settling  of  the  crust  or  its  movement  on  a  large 
scale  is  delayed.  Perhaps  the  delay  continues  until  the 
stresses  become  so  great  that  of  themselves  they  over- 
come the  inertia,  or  possibly  some  outside  agency,  whose 
nature  we  shall  consider  later,  reenforces  the  stresses  and 
gives  the  slight  impulse  which  is  enough  to  release  them 
and  allow  the  earth's  crust  to  settle  into  a  new  state  of 
equilibrium.  When  contraction  proceeds  actively,  the 
ocean  segments,  being  largest  and  heaviest,  are  likely  to 
settle  most,  resulting  in  a  deepening  of  the  oceans  and  an 
emergence  of  the  lands.  Following  each  considerable  con- 
traction there  would  be  an  increase  in  the  speed  of  rota- 
tion. The  repeated  contractions  with  consequent  growth 
of  the  equatorial  bulge  would  alternate  with  long  quiet 
periods  during  which  tidal  retardation  would  again  de- 
crease the  speed  of  rotation  and  hence  lessen  the  bulge. 
The  result  would  be  repeated  changes  of  distribution  of 
land  and  water,  with  consequent  changes  in  climate. 

12  T.  C.  Chamberlin:   Diastrophism,  the  Ultimate  Basis  of  Correlation; 
Jour.  Geol.,  Vol.  16,  1909;  Chas.  Schuchert:  loc.  cit. 


196  CLIMATIC  CHANGES 

I.  We  shall  now  consider  the  climatic  effect  of  the 
repeated  changes  in  the  relative  amounts  of  land  and 
water  which  appear  to  have  resulted  from  the  earth's 
contraction  and  from  changes  in  its  speed  of  rotation. 
During  many  geologic  epochs  a  larger  portion  of  the 
earth  was  covered  with  water  than  at  present.  For  ex- 
ample, during  at  least  twelve  out  of  about  twenty  epochs, 
North  America  has  suffered  extensive  inundations,13  and 
in  general  the  extensive  submergence  of  Europe,  the 
other  area  well  known  geologically,  has  coincided  with 
that  of  North  America.  At  other  times,  the  ocean  has 
been  less  extensive  than  now,  as  for  example  during  the 
recent  glacial  period,  and  probably  during  several  of 
the  glacial  periods  of  earlier  date.  Each  of  the  numerous 
changes  in  the  relative  extent  of  the  lands  must  have 
resulted  in  a  modification  of  climate.1*  This  modification 
would  occur  chiefly  because  water  becomes  warm  far 
more  slowly  than  land,  and  cools  off  far  more  slowly. 

An  increase  in  the  lands  would  cause  changes  in  several 
climatic  conditions,  (a)  The  range  of  temperature  be- 
tween day  and  night  and  between  summer  and  winter 
would  increase,  for  lands  become  warmer  by  day  and  in 
summer  than  do  oceans,  and  cooler  at  night  and  in 
winter.  The  higher  summer  temperature  when  the  lands 
are  widespread  is  due  chiefly  to  the  fact  that  the  land,  if 
not  snow-covered,  absorbs  more  of  the  sun's  radiant 
energy  than  does  the  ocean,  for  its  reflecting  power  is 
low.  The  lower  winter  temperature  when  lands  are  wide- 
spread occurs  not  only  because  they  cool  off  rapidly  but 

is  Pirsson-Schuchert :  Textbook  of  Geology,  1915,  Vol.  II,  p.  982;  Chas. 
Schuchert:  Paleogeography  of  North  America;  Bull.  Geol.  Soc.  Am.,  Vol. 
20,  pp.  427-606;  reference  on  p.  499. 

1*  The  general  subject  of  the  climatic  significance  of  continentality  is 
discussed  by  C.  E.  P.  Brooks:  Continentality  and  Temperature;  Quart. 
Jour.  Eoyal  Meteorol.  Soc.,  April,  1917,  and  Oct.,  1918. 


TERRESTRIAL  CAUSES  OF  CHANGES         197 

because  the  reduced  oceans  cannot  give  them  so  much 
heat.  Moreover,  the  larger  the  land,  the  more  generally 
do  the  winds  blow  outward  from  it  in  winter  and  thus 
prevent  the  ocean  heat  from  being  carried  inland.  So 
long  as  the  ocean  is  not  frozen  in  high  latitudes,  it  is 
generally  the  chief  source  of  heat  in  winter,  for  the  nights 
are  several  months  long  near  the  poles,  and  even  when 
the  sun  does  shine  its  angle  is  so  low  that  reflection  from 
the  snow  is  very  great.  Furthermore,  although  on  the 
average  there  is  more  reflection  from  water  than  from 
land,  the  opposite  is  true  in  high  latitudes  in  winter 
when  the  land  is  snow-covered  while  the  ocean  is  rela- 
tively dark  and  is  roughened  by  the  waves.  Another 
factor  in  causing  large  lands  to  have  extremely  low  tem- 
perature in  winter  is  the  fact  that  in  proportion  to  their 
size  they  are  less  protected  by  fog  and  cloud  than  are 
smaller  areas.  The  belt  of  cloud  and  fog  which  is  usually 
formed  when  the  wind  blows  from  the  ocean  to  the  rela- 
tively cold  land  is  restricted  to  the  coastal  zone.  Thus  the 
larger  the  land,  the  smaller  the  fraction  in  which  loss  of 
heat  by  radiation  is  reduced  by  clouds  and  fogs.  Hence 
an  increase  in  the  land  area  is  accompanied  by  an  in- 
crease in  the  contrasts  in  temperature  between  land  and 
water. 

(b)  The  contrasts  in  temperature  thus  produced  must 
cause  similar  contrasts  in  atmospheric  pressure,  and 
hence  stronger  barometric  gradients,  (c)  The  strong 
gradients  would  mean  strong  winds,  flowing  from  land 
to  sea  or  from  sea  to  land,  (d)  Local  convection  would 
also  be  strengthened  in  harmony  with  the  expansion  of 
the  lands,  for  the  more  rapid  heating  of  land  than  of 
water  favors  active  convection. 

(e)  As  the  extent  of  the  ocean  diminished,  there  would 
normally  be  a  decrease  in  the  amount  of  water  vapor  for 


198  CLIMATIC  CHANGES 

three  reasons:  (1)  Evaporation  from  the  ocean  is  the 
great  source  of  water  vapor.  Other  conditions  being 
equal,  the  smaller  the  ocean  becomes,  the  less  the  evapo- 
ration. (2)  The  amount  of  water  vapor  in  the  air  dimin- 
ishes as  convection  increases,  since  upward  convection 
is  a  chief  method  by  which  condensation  and  precipita- 
tion are  produced,  and  water  vapor  removed  from  the 
atmosphere.  (3)  Nocturnal  cooling  sufficient  to  produce 
dew  and  frost  is  very  much  more  common  upon  land  than 
upon  the  ocean.  The  formation  of  dew  and  frost  dimin- 
ishes the  amount  of  water  vapor  at  least  temporarily. 
(f)  Any  diminution  in  water  vapor  produced  in  these 
ways,  or  otherwise,  is  significant  because  water  vapor  is 
the  most  essential  part  of  the  atmosphere  so  far  as  regu- 
lation of  temperature  is  concerned.  It  tends  to  keep  the 
days  from  becoming  hot  or  the  nights  cold.  Therefore 
any  decrease  in  water  vapor  would  increase  the  diurnal 
and  seasonal  range  of  temperature,  making  the  climate 
more  extreme  and  severe.  Thus  a  periodic  increase  in  the 
area  of  the  continents  would  clearly  make  for  periodic 
increased  climatic  contrasts,  with  great  extremes,  a  type 
of  climatic  change  which  has  recurred  again  and  again. 
Indeed,  each  great  glaciation  accompanied  or  followed 
extensive  emergence  of  the  lands.15 

Whether  or  not  there  has  been  a  progressive  increase 
from  era  to  era  in  the  area  of  the  lands  is  uncertain. 
Good  authorities  disagree  widely.  There  is  no  doubt, 
however,  that  at  present  the  lands  are  more  extensive 
than  at  most  times  in  the  past,  though  smaller,  perhaps, 
than  at  certain  periods.  The  wide  expanse  of  lands  helps 
explain  the  prominence  of  seasons  at  present  as  com- 
pared with  the  past. 


Schuchert:  Climates  of  Geologic  Time;  in  The  Climatic  Factor; 
Carnegie  Institution,  1914,  p.  286. 


TERRESTRIAL  CAUSES  OF  CHANGES         199 

II.  The  contraction  of  the  earth,  as  we  have  seen,  has 
produced  great  changes  in  the  distribution  as  well  as  in 
the  extent  of  land  and  water.  Large  parts  of  the  present 
continents  have  been  covered  repeatedly  by  the  sea,  and 
extensive  areas  now  covered  with  water  have  been  land. 
In  recent  geological  times,  that  is,  during  the  Pliocene  and 
Pleistocene,  much  of  the  present  continental  shelf,  the 
zone  less  than  600  feet  below  sea  level,  was  land.  If  the 
whole  shelf  had  been  exposed,  the  lands  would  have  been 
greater  than  at  present  by  an  area  larger  than  North 
America.  When  the  lands  were  most  elevated,  or  a  little 
earlier,  North  America  was  probably  connected  with 
Asia  and  almost  with  Europe.  Asia  in  turn  was  appar- 
ently connected  with  the  larger  East  Indian  islands.  In 
much  earlier  times  land  occupied  regions  where  now  the 
ocean  is  fairly  deep.  Groups  of  islands,  such  as  the  East 
Indies  and  Malaysia  and  perhaps  the  West  Indies,  were 
united  into  widespreading  land  masses.  Figs.  7  and  9, 
illustrating  the  paleography  of  the  Permian  and  the 
Cretaceous  periods,  respectively,  indicate  a  land  distri- 
bution radically  different  from  that  of  today. 

So  far  as  appears  from  the  scattered  facts  of  geologi- 
cal history,  the  changes  in  the  distribution  of  land  seem 
to  have  been  marked  by  the  following  characteristics :  (1) 
Accompanying  the  differentiation  of  continental  and 
oceanic  segments  of  the  earth's  crust,  the  oceans  have 
become  somewhat  deeper,  and  their  basins  perhaps 
larger,  while  the  continents,  on  the  average,  have  been 
more  elevated  and  less  subject  to  submergence.  Hence 
there  have  been  less  radical  departures  from  the  present 
distribution  during  the  relatively  recent  Cenozoic  era 
than  in  the  ancient  Paleozoic  because  the  submergence  of 
continental  areas  has  become  less  general  and  less  fre- 
quent. For  example,  the  last  extensive  epeiric  or  interior 


200  CLIMATIC  CHANGES 

sea  in  North  America  was  in  the  Cretaceous,  at  least  ten 
million  years  ago,  and  according  to  Barrell  perhaps  fifty 
million,  while  in  Europe,  according  to  de  Lapparent,16  a 
smaller  share  of  the  present  continent  has  been  sub- 
merged since  the  Cretaceous  than  before.  Indeed,  as  in 
North  America,  the  submergence  has  decreased  on  the 
average  since  the  Paleozoic  era.  (2)  The  changes  in  dis- 
tribution of  land  which  have  taken  place  during  earth 
history  have  been  cyclic.  Repeatedly,  at  the  close  of  each 
of  the  score  or  so  of  geologic  periods,  the  continents 
emerged  more  or  less,  while  at  the  close  of  the  groups  of 
periods  known  as  eras,  the  lands  were  especially  large 
and  emergent.  After  each  emergence,  a  gradual  encroach- 
ment of  the  sea  took  place,  and  toward  the  close  of  sev- 
eral of  the  earlier  periods,  the  sea  appears  to  have 
covered  a  large  fraction  of  the  present  land  areas.  (3)  On 
the  whole,  the  amount  of  land  in  the  middle  and  high  lati- 
tudes of  the  northern  hemisphere  appears  to  have  in- 
creased during  geologic  time.  Such  an  increase  does  not 
require  a  growth  of  the  continents,  however,  in  the 
broader  sense  of  the  term,  but  merely  that  a  smaller 
fraction  of  the  continent  and  its  shelf  should  be  sub- 
merged. (4)  In  tropical  latitudes,  on  the  other  hand,  the 
extent  of  the  lands  seems  to  have  decreased,  apparently 
by  the  growth  of  the  ocean  basins.  South  America  and 
Africa  are  thought  by  many  students  to  have  been  con- 
nected, and  Africa  was  united  with  India  via  Mada- 
gascar, as  is  suggested  in  Fig.  9.  The  most  radical  cyclic 
as  well  as  the  most  radical  progressive  changes  in  land 
distribution  also  seem  to  have  taken  place  in  tropical 
regions.17 
Although  there  is  much  evidence  of  periodic  increase 

is  A.  de  Lapparent :  Traite  de  Geologic,  1906. 

i?  Chas.  Schuchert :  Historical  Geology,  1915,  p.  464. 


202  CLIMATIC  CHANGES 

of  the  sea  in  equatorial  latitudes  and  of  land  in  high  lati- 
tudes, it  has  remained  for  the  zoologist  Metcalf  to  pre- 
sent a  very  pretty  bit  of  evidence  that  at  certain  times 
submergence  along  the  equator  coincided  with  emergence 
in  high  latitudes,  and  vice  versa.  Certain  fresh  -water 
frogs  which  carry  the  same  internal  parasite  are  confined 
to  two  widely  separated  areas  in  tropical  and  south  tem- 
perate America  and  in  Australia.  The  extreme  improba- 
bility that  both  the  frogs  and  the  parasites  could  have 
originated  independently  in  two  unconnected  areas  and 
could  have  developed  by  convergent  evolution  so  that 
they  are  almost  identical  in  the  two  continents  makes  it 
almost  certain  that  there  must  have  been  a  land  con- 
nection between  South  America  and  Australia,  presum- 
ably by  way  of  Antarctica.  The  facts  as  to  the  parasites 
seem  also  to  prove  that  while  the  land  connection  existed 
there  was  a  sea  across  South  America  in  equatorial  lati- 
tudes. The  parasite  infests  not  only  the  frogs  but  the 
American  toads  known  as  Bufo.  Now  Bufo  originated 
north  of  the  equator  in  America  and  differs  from  the 
frogs  which  originated  in  southern  South  America  in 
not  being  found  in  Australia.  This  raises  the  question  of 
how  the  frogs  could  go  to  Australia  via  Antarctica  carry- 
ing the  parasite  with  them,  while  the  toads  could  not  go. 
Metcalf 's  answer  is  that  the  toads  were  cut  off  from  the 
southern  part  of  South  America  by  an  equatorial  sea 
until  after  the  Antarctic  connection  between  the  Old 
World  and  the  New  was  severed. 

As  Patagonia  let  go  of  Antarctica  by  subsidence  of  the  inter- 
vening land  area,  there  was  a  probable  concomitant  rise  of  land 
through  what  is  now  middle  South  America  and  the  northern 
and  southern  portions  of  this  continent  came  together.18 

is  M.  M.  Metcalf :  Upon  an  important  method  of  studying  problems  of 
relationship  and  of  geographical  distribution;  Proceedings  National  Acad- 
emy of  Sciences,  Vol.  6,  July,  1920,  pp.  432-433. 


TERRESTRIAL  CAUSES  OF  CHANGES         203 

These  various  changes  in  the  earth's  crust  have  given 
rise  to  certain  specific  types  of  distribution  of  the  lands, 
which  will  now  be  considered.  We  shall  inquire  what  cli- 
matic conditions  would  arise  from  changes  in  (a)  the 
continuity  of  the  lands  from  north  to  south,  (b)  the 
amount  of  land  in  tropical  latitudes,  and  (c)  the  amount 
of  land  in  middle  and  high  latitudes. 

(a)  At  present  the  westward  drift  of  warm  waters,  set 
in  motion  by  the  trade  winds,  is  interrupted  by  land 
masses  and  turned  poleward,  producing  the  important 
Gulf  Stream  Drift  and  Japan  Current  in  the  northern 
hemisphere,  and  corresponding,  though  less  important, 
currents  in  the  southern  hemisphere.  During  the  past, 
quite  different  sets  of  ocean  currents  doubtless  have 
existed  in  response  to  a  different  distribution  of  land. 
Repeatedly,  in  the  mid-Cretaceous  (Fig.  9)  and  several 
other  periods,  the  present  American  barrier  to  the  west- 
ward-moving tropical  current  was  broken  in  Central 
America.  Even  if  the  supposed  continent  of  *  *  Gondwana 
Land"  extended  from  Africa  to  South  America  in  equa- 
torial latitudes,  strong  currents  must  still  have  flowed 
westward  along  its  northern  shore  under  the  impulse  of 
the  peculiarly  strong  trade  winds  which  the  equatorial 
land  would  create.  Nevertheless  at  such  times  relatively 
little  warm  tropical  water  presumably  entered  the  North 
Atlantic,  for  it  escaped  into  the  Pacific.  At  several  other 
times,  such  as  the  late  Ordovician  and  mid-Devonian, 
when  the  isthmian  barrier  existed,  it  probably  turned  an 
important  current  northward  into  what  is  now  the  Mis- 
sissippi Basin  instead  of  into  the  Atlantic.  There  it 
traversed  an  epeiric,  or  mid-continental  sea  open  to  both 
north  and  south.  Hence  its  effectiveness  in  warming 
Arctic  regions  must  have  been  quite  different  from  that 
of  the  present  Gulf  Stream. 


204  CLIMATIC  CHANGES 

(b)  We  will  next  consider  the  influences  of  changes  in 
the  amount  of  equatorial  and  tropical  land.  As  such  lands 
are  much  hotter  than  the  corresponding  seas,  the  inten- 
sity and  width  of  the  equatorial  belt  of  low  pressure  must 
be  great  when  they  are  extensive.  Hence  the  trade  winds 
must  have  been  stronger  than  now  whenever  tropical 
lands  were  more  extensive  than  at  present.  This  is  be- 
cause the  trades  are  produced  by  the  convection  due  to 
excessive  heat  along  the  heat  equator.  There  the  air 
expands  upward  and  flows  poleward  at  high  altitudes. 
The  trade  wind  consists  of  air  moving  toward  the  heat 
equator  to  take  the  place  of  the  air  which  there  rises. 
When  the  lands  in  low  latitudes  were  wide  the  trade 
winds  must  also  have  dominated  a  wide  belt.  The  greater 
width  of  the  trade-wind  belt  today  over  Africa  than  over 
the  Atlantic  illustrates  the  matter.  The  belt  must  have 
been  still  wider  when  Gondwana  Land  was  large,  as  it  is 
believed  to  have  been  during  the  Paleozoic  era  and  the 
early  Mesozoic. 

An  increase  in  the  width  of  the  equatorial  belt  of 
low  pressure  under  the  influence  of  broad  tropical  lands 
would  be  accompanied  not  only  by  stronger  and  more 
widespread  trade  winds,  but  by  a  corresponding  strength- 
ening of  the  subtropical  belts  of  high  pressure.  The  chief 
reason  would  be  the  greater  expansion  of  the  air  in  the 
equatorial  low  pressure  belt  and  the  consequent  more 
abundant  outflow  of  air  at  high  altitudes  in  the  form  of 
anti-trades  or  winds  returning  poleward  above  the  trades. 
Such  winds  would  pile  up  the  air  in  the  region  of  the  high- 
pressure  belt.  Moreover,  since  the  meridians  converge  as 
one  proceeds  away  from  the  equator,  the  air  of  the  pole- 
ward-moving anti-trades  tends  to  be  crowded  as  it 
reaches  higher  latitudes,  thus  increasing  the  pressure. 
Unless  there  were  a  corresponding  increase  in  tropical 


TERRESTRIAL  CAUSES  OF  CHANGES         205 

cyclones,  one  of  the  most  prominent  results  of  the 
strengthened  trades  and  the  intensified  subtropical  high- 
pressure  belt  at  times  of  broad  lands  in  low  latitudes 
would  be  great  deserts.  It  will  be  recalled  that  the  trade- 
wind  lowlands  and  the  extra-tropical  belt  of  highs  are  the 
great  desert  belts  at  present.  The  trade-wind  lowlands 
are  desert  because  air  moving  into  warmer  latitudes 
takes  up  water  except  where  it  is  cooled  by  rising  on 
mountain-sides.  The  belt  of  highs  is  arid  because  there, 
too,  air  is  being  warmed,  but  in  this  case  by  descending 
from  aloft. 

Again,  if  the  atmospheric  pressure  in  the  subtropical 
belt  should  be  intensified,  the  winds  flowing  poleward 
from  this  belt  would  necessarily  become  stronger.  These 
would  begin  as  southwesterlies  in  the  northern  hemi- 
sphere and  northwesterlies  in  the  southern.  In  the  pre- 
ceding chapter  we  have  seen  that  such  winds,  especially 
when  cyclonic  storms  are  few  and  mild,  are  a  powerful 
agent  in  transferring  subtropical  heat  poleward.  If  the 
strength  of  the  westerlies  were  increased  because  of 
broad  lands  in  low  latitudes,  their  efficacy  in  transferring 
heat  would  be  correspondingly  augmented.  It  is  thus 
evident  that  any  change  in  the  extent  of  tropical  lands 
during  the  geologic  past  must  have  had  important  cli- 
matic consequences  in  changing  the  velocity  of  the 
atmospheric  circulation  and  in  altering  the  transfer  of 
heat  from  low  latitudes  to  high.  When  the  equatorial  and 
tropical  lands  were  broad  the  winds  and  currents  must 
have  been  strong,  much  heat  must  have  been  carried 
away  from  low  latitudes,  and  the  contrast  between  low 
and  high  latitudes  must  have  been  relatively  slight.  As 
we  have  already  remarked,  leading  paleogeographers 
believe  that  changes  in  the  extent  of  the  lands  have  been 
especially  marked  in  low  latitudes,  and  that  on  the  aver- 


206  CLIMATIC  CHANGES 

age  there  has  been  a  decrease  in  the  extent  of  land  within 
the  tropics.  Gondwana  Land  is  the  greatest  illustration 
of  this.  In  the  same  way,  on  the  numerous  paleogeo- 
graphic  maps  of  North  America,  most  paleogeographers 
have  shown  fairly  extensive  lands  south  of  the  latitude  of 
the  United  States  during  most  of  the  geologic  epochs.19 

(c)  There  is  evidence  that  during  geologic  history  the 
area  of  the  lands  in  middle  and  high  latitudes,  as  well  as 
in  low  latitudes,  has  changed  radically.  An  increase  in 
such  lands  would  cause  the  winters  to  grow  colder.  This 
would  be  partly  because  of  the  loss  of  heat  by  radiation 
into  the  cold  dry  air  over  the  continents  in  winter,  and 
partly  because  of  increased  reflection  from  snow  and 
frost,  which  gather  much  more  widely  upon  the  land  than 
upon  the  ocean.  Furthermore,  in  winter  when  the  conti- 
nents are  relatively  cold,  there  is  a  strong  tendency  for 
winds  to  blow  out  from  the  continent  toward  the  ocean. 
The  larger  the  land  the  stronger  this  tendency.  In  Asia 
it  gives  rise  to  strong  winter  monsoons.  The  effect  of 
such  winds  is  illustrated  by  the  way  in  which  the  wester- 
lies prevent  the  Gulf  Stream  from  warming  the  eastern 
United  States  in  winter.  The  Gulf  Stream  warms  north- 
western Europe  much  more  than  the  United  States  be- 
cause, in  Europe,  the  prevailing  winds  are  onshore. 

Another  effect  of  an  increase  in  the  area  of  the  lands  in 
middle  and  high  latitudes  would  be  to  interpose  bar- 
riers to  oceanic  circulation  and  thus  lower  the  tempera- 
ture of  polar  regions.  This  would  not  mean  glaciation  in 
high  latitudes,  however,  even  when  the  lands  were  wide- 
spread as  in  the  Mesozoic  and  early  Tertiary.  Students 
of  glaciology  are  more  and  more  thoroughly  convinced 

i»Chas.  Schuchert:  Paleogeography  of  North  America;  Bull.  Geol.  Soc. 
Am.,  Vol.  20,  1910;  and  Willis,  Salisbury,  and  others:  Outlines  of  Geologic 
History,  1910. 


TERRESTRIAL  CAUSES  OF  CHANGES         207 

that  glaciation  depends  on  the  availability  of  moisture 
even  more  than  upon  low  temperature. 

In  conclusion  it  may  be  noted  that  each  of  the  several 
climatic  influences  of  increased  land  area  in  the  high 
latitudes  would  tend  to  increase  the  contrasts  between 
land  and  sea,  between  winter  and  summer,  and  between 
low  latitudes  and  high.  In  other  words,  so  far  as  the 
effect  upon  high  latitudes  themselves  is  concerned,  an 
expansion  of  the  lands  there  would  tend  in  the  same 
direction  as  a  diminution  in  low  latitudes.  In  so  far  as 
the  general  trend  of  geological  evolution  has  been  toward 
more  land  in  high  latitudes  and  less  in  low,  it  would  help 
to  produce  a  progressive  increase  in  climatic  diversity 
such  as  is  faintly  indicated  in  the  rock  strata.  On  the 
other  hand,  the  oscillations  in  the  distribution  of  the 
lands,  of  which  geology  affords  so  much  evidence,  must 
certainly  have  played  an  important  part  in  producing  the 
periodic  changes  of  climate  which  the  earth  has  under- 
gone. 

III.  Throughout  geological  history  there  is  abundant 
evidence  that  the  process  of  contraction  has  led  to 
marked  differences  not  only  in  the  distribution  and  area 
of  the  lands,  but  in  their  height.  On  the  whole  the  lands 
have  presumably  increased  in  height  since  the  Protero- 
zoic,  somewhat  in  proportion  to  the  increased  differentia- 
tion of  continents  and  oceans.20  If  there  has  been  such  an 
increase,  the  contrast  between  the  climate  of  ocean  and 
land  must  have  been  accentuated,  for  highlands  have  a 
greater  diurnal  and  seasonal  range  of  temperature  than 
do  lowlands.  The  ocean  has  very  little  range  of  either 
sort.  The  large  range  at  high  altitudes  is  due  chiefly  to 
the  small  quantity  of  water  vapor,  for  this  declines 

20  Chas.  Schuchert :  The  Earth 's  Changing  Surface  and  Climate ;  in  Lull : 
The  Evolution  of  the  Earth  and  Its  Inhabitants,  1918,  p.  50. 


208  CLIMATIC  CHANGES 

steadily  with  increased  altitude.  A  diminution  in  the 
density  of  the  other  constituents  of  the  air  also  decreases 
the  blanketing  effect  of  the  atmosphere.  In  conformity 
with  the  great  seasonal  range  in  temperature  at  times 
when  the  lands  stand  high,  the  direction  of  the  wind 
would  be  altered.  When  the  lands  are  notably  warmer 
than  the  oceans,  the  winds  commonly  flow  from  land  to 
sea,  and  when  the  continents  are  much  colder  than  the 
oceans,  the  direction  is  reversed.  The  monsoons  of  Asia 
are  examples.  Strong  seasonal  winds  disturb  the  normal 
planetary  circulation  of  the  trade  winds  in  low  latitudes 
and  of  the  westerlies  in  middle  latitudes.  They  also  inter- 
fere with  the  ocean  currents  set  in  motion  by  the  planet- 
ary winds.  The  net  result  is  to  hinder  the  transfer  of 
heat  from  low  latitudes  to  high,  and  thus  to  increase  the 
contrasts  between  the  zones.  Local  as  well  as  zonal  con- 
trasts are  also  intensified.  The  higher  the  land,  the 
greater,  relatively  speaking,  are  the  cloudiness  and  pre- 
cipitation on  seaward  slopes,  and  the  drier  the  interior. 
Indeed,  most  highlands  are  arid.  Henry's21  recent  study 
of  the  vertical  distribution  of  rainfall  on  mountain-sides 
indicates  that  a  decrease  sets  in  at  about  3500  feet  in  the 
tropics  and  only  a  little  higher  in  mid-latitudes. 

In  addition  to  the  main  effects  upon  atmospheric  cir- 
culation and  precipitation,  each  of  the  many  upheavals 
of  the  lands  must  have  been  accompanied  by  many  minor 
conditions  which  tended  toward  diversity.  For  example, 
the  streams  were  rejuvenated,  and  instead  of  meandering 
perhaps  over  vast  flood  plains  they  intrenched  their 
channels  and  in  many  cases  dug  deep  gorges.  The  water 
table  was  lowered,  soil  was  removed  from  considerable 
areas,  the  bare  rock  was  exposed,  and  the  type  of  domi- 

21  A.  J.  Henry:  The  Decrease  of  Precipitation  with  Altitude;  Monthly 
Weather  Eeview,  Vol.  47,  1919,  pp.  33-41. 


TERRESTRIAL  CAUSES  OF  CHANGES         209 

nant  vegetation  altered  in  many  places.  An  almost  barren 
ridge  may  represent  all  that  remains  of  what  was  once  a 
vast  forested  flood  plain.  Thus,  increased  elevation  of  the 
land  produces  contrasted  conditions  of  slope,  vegetation, 
availability  of  ground  water,  exposure  to  wind  and  so 
forth,  and  these  unite  in  diversifying  climate.  Where 
mountains  are  formed,  strong  contrasts  are  sure  to 
occur.  The  windward  slopes  may  be  very  rainy,  while 
neighboring  leeward  slopes  are  parched  by  a  dry  foehn 
wind.  At  the  same  time  the  tops  may  be  snow-covered. 
Increased  local  contrasts  in  climatic  conditions  are 
known  to  influence  the  intensity  of  cyclonic  storms,22  and 
these  affect  the  climatic  conditions  of  all  middle  and  high 
latitudes,  if  not  of  the  entire  earth.  The  paths  followed 
by  cyclonic  storms  are  also  altered  by  increased  contrast 
between  land  and  water.  "When  the  continents  are  notably 
colder  than  the  neighboring  oceans,  high  atmospheric 
pressure  develops  on  the  lands  and  interferes  with  the 
passage  of  lows,  which  are  therefore  either  deflected 
around  the  continent  or  forced  to  move  slowly. 

The  distribution  of  lofty  mountains  has  an  even  more 
striking  climatic  effect  than  the  general  uplift  of  a  region. 
In  Proterozoic  times  there  was  a  great  range  in  the  Lake 
Superior  region ;  in  the  late  Devonian  the  Acadian  moun- 
tains of  New  England  and  the  Maritime  Provinces  of 
Canada  possibly  attained  a  height  equal  to  the  present 
Eockies.  Subsequently,  in  the  late  Paleozoic  a  significant 
range  stood  where  the  Ouachitas  now  are.  Accompanying 
the  uplift  of  each  of  these  ranges,  and  all  others,  the 
climate  of  the  surrounding  area,  especially  to  leeward, 
must  have  been  altered  greatly.  Many  extensive  salt  de- 

22Chas.  F.  Brooks:  Monthly  Weather  Eeview,  Vol.  46,  1918,  p.  511;  and 
also  A.  J.  Henry  and  others:  Weather  Forecasting  in  the  United  States, 
1913. 


210  CLIMATIC  CHANGES 

posits  found  now  in  fairly  humid  regions,  for  example, 
the  Pennsylvanian  and  Permian  deposits  of  Kansas  and 
Oklahoma,  were  probably  laid  down  in  times  of  local 
aridity  due  to  the  cutting  off  of  moisture-bearing  winds 
by  the  mountains  of  Llanoria  in  Louisiana  and  Texas. 
Hence  such  deposits  do  not  necessarily  indicate  periods 
of  widespread  and  profound  aridity. 

When  the  causes  of  ancient  glaciation  were  first  con- 
sidered by  geologists,  about  the  middle  of  the  nineteenth 
century,  it  was  usually  assumed  that  the  glaciated  areas 
had  been  elevated  to  great  heights,  and  thus  rendered 
cold  enough  to  permit  the  accumulation  of  glaciers.  The 
many  glaciers  occurring  in  the  Alps  of  central  Europe 
where  glaciology  arose  doubtless  suggested  this  explana- 
tion. However,  it  is  now  known  that  most  of  the  ancient 
glaciation  was  not  of  the  alpine  type,  and  there  is  ade- 
quate proof  that  the  glacial  periods  cannot  be  explained 
as  due  directly  and  solely  to  uplift.  Nevertheless,  up- 
heavals of  the  lands  are  among  the  most  important  fac- 
tors in  controlling  climate,  and  variations  in  the  height 
of  the  lands  have  doubtless  assisted  in  producing  climate 
oscillations,  especially  those  of  long  duration.  Moreover, 
the  progressive  increase  in  the  height  of  the  lands  has 
presumably  played  a  part  in  fostering  local  and  zonal 
diversity  in  contrast  with  the  relative  uniformity  of 
earlier  geological  times. 

IV.  The  contraction  of  the  earth  has  been  accompanied 
by  volcanic  activity  as  well  as  by  changes  in  the  extent, 
distribution,  and  altitude  of  the  lands.  The  probable  part 
played  by  volcanic  dust  as  a  contributory  factor  in  pro- 
ducing short  sudden  climatic  variations  has  already  been 
discussed.  There  is,  however,  another  though  probably 
less  important  respect  in  which  volcanic  activity  may 
have  had  at  least  a  slight  climatic  significance.  The  oldest 


TERRESTRIAL  CAUSES  OF  CHANGES         211 

known  rocks,  those  of  the  Archean  era,  contain  so  much 
igneous  matter  that  many  students  have  assumed  that 
they  show  that  the  entire  earth  was  once  liquid.  It  is  now 
considered  that  they  merely  indicate  igneous  activity  of 
great  magnitude.  In  the  later  part  of  Proterozoic  time, 
during  the  second  quarter  of  the  earth's  history  accord- 
ing to  Schuchert's  estimate,  there  were  again  vast  out- 
flowings  of  lava.  In  the  Lake  Superior  district,  for  ex- 
ample, a  thickness  of  more  than  a  mile  accumulated  over 
a  large  area,  and  lavas  are  common  in  many  areas  where 
rocks  of  this  age  are  known.  The  next  quarter  of  the 
earth's  history  elapsed  without  any  correspondingly 
great  outflows  so  far  as  is  known,  though  several  lesser 
ones  occurred.  Toward  the  end  of  the  last  quarter,  and 
hence  quite  recently  from  the  geological  standpoint, 
another  period  of  outflows,  perhaps  as  noteworthy  as 
that  of  the  Proterozoic,  occurred  in  the  Cretaceous  and 
Tertiary. 

The  climatic  effects  of  such  extensive  lava  flows  would 
be  essentially  as  follows :  In  the  first  place  so  long  as  the 
lavas  were  hot  they  would  set  up  a  local  system  of  con- 
vection with  inflowing  winds.  This  would  interfere  at 
least  a  little  with  the  general  winds  of  the  area.  Again, 
where  the  lava  flowed  out  into  water,  or  where  rain  fell 
upon  hot  lava,  there  would  be  rapid  evaporation  which 
would  increase  the  rainfall.  Then  after  the  lava  had 
cooled,  it  would  still  influence  climate  a  trifle  in  so  far  as 
its  color  was  notably  darker  or  lighter  than  that  of  the 
average  surface.  Dark  surfaces  absorb  solar  heat  and 
become  relatively  warm  when  the  sun  shines  upon  them. 
Dark  objects  likewise  radiate  heat  more  rapidly  than 
light-colored  objects.  Hence  they  cool  more  rapidly  at 
night,  and  in  the  winter.  As  most  lavas  are  relatively 
dark  they  increase  the  average  diurnal  range  of  tempera- 


212  CLIMATIC  CHANGES 

ture.  Hence  even  after  they  are  cool  they  increase  the 
climatic  diversity  of  the  land. 

The  amount  of  heat  given  to  the  atmosphere  by  an 
extensive  lava  flow,  though  large  according  to  human 
standards,  is  small  compared  with  the  amount  received 
from  the  sun  by  a  like  area,  except  during  the  first  few 
weeks  or  months  before  the  lava  has  formed  a  thick 
crust.  Furthermore,  probably  only  a  small  fraction  of 
any  large  series  of  flows  occurred  in  a  given  century  or 
millennium.  Moreover,  even  the  largest  lava  flows  covered 
an  area  of  only  a  few  hundredths  of  one  per  cent  of  the 
earth's  surface.  Nevertheless,  the  conditions  which  mod- 
ify climate  are  so  complicated  that  it  would  be  rash  to 
state  that  this  amount  of  additional  heat  has  been  of 
no  climatic  significance.  Like  the  proverbial  ' '  straw  that 
broke  the  camel's  back,"  the  changes  it  would  surely 
produce  in  local  convection,  atmospheric  pressure,  and 
the  direction  of  the  wind  may  have  helped  to  shift  the 
paths  of  storms  and  to  produce  other  complications  which 
were  of  appreciable  climatic  significance. 

V.  The  last  point  which  we  shall  consider  in  connection 
with  the  effect  of  the  earth's  interior  upon  climate  is 
internal  heat.  The  heat  given  off  by  lavas  is  merely  a 
small  part  of  that  which  is  emitted  by  the  earth  as  a 
whole.  In  the  earliest  part  of  geological  history  enough 
heat  may  have  escaped  from  the  interior  of  the  earth  to 
exert  a  profound  influence  on  the  climate.  Knowlton,23 
as  we  have  seen,  has  recently  built  up  an  elaborate  theory 
on  this  assumption.  At  present,  however,  accurate  meas- 
urements show  that  the  escape  of  heat  is  so  slight  that 
it  has  no  appreciable  influence  except  in  a  few  volcanic 

23  F.  H.  Knowlton:  Evolution  of  Geologic  Climates;  Bull.  Geol.  Soc.  Am., 
Vol.  30,  Dec.,  1919,  pp.  499-566. 


TERRESTRIAL  CAUSES  OF  CHANGES         213 

areas.  It  is  estimated  to  raise  the  average  temperature 
of  the  earth's  surface  less  than  0.1°C.24 

In  order  to  contribute  enough  heat  to  raise  the  surface 
temperature  1°C.,  the  temperature  gradient  from  the 
interior  of  the  earth  to  the  surface  would  need  to  be  ten 
times  as  great  as  now,  for  the  rate  of  conduction  varies 
directly  with  the  gradient.  If  the  gradient  were  ten  times 
as  great  as  now,  the  rocks  at  a  depth  of  two  and  one-half 
miles  would  be  so  hot  as  to  be  almost  liquid  according  to 
Barrell's25  estimates.  The  thick  strata  of  unmetamor- 
phosed  Paleozoic  rocks  indicate  that  such  high  tempera- 
tures have  not  prevailed  at  such  slight  depths  since  the 
Proterozoic.  Furthermore,  the  fact  that  the  climate  was 
cold  enough  to  permit  glaciation  early  in  the  Proterozoic 
era  and  at  from  one  to  three  other  times  before  the  open- 
ing of  the  Paleozoic  suggests  that  the  rate  of  escape  of 
heat  was  not  rapid  even  in  the  first  half  of  the  earth's 
recorded  history.  Yet  even  if  the  general  escape  of  heat 
has  never  been  large  since  the  beginning  of  the  better- 
known  part  of  geological  history,  it  was  presumably 
greater  in  early  times  than  at  present. 

If  there  actually  has  been  an  appreciable  decrease  in 
the  amount  of  heat  given  out  by  the  earth's  interior,  its 
effects  would  agree  with  the  observed  conditions  of  the 
geological  record.  It  would  help  to  explain  the  relative 
mildness  of  zonal,  seasonal,  and  local  contrasts  of  climate 
in  early  geological  times,  but  it  would  not  help  to  explain 
the  long  oscillations  from  era  to  era  which  appear  to  have 
been  of  much  greater  importance.  Those  oscillations,  so 
far  as  we  can  yet  judge,  may  have  been  due  in  part  to 
solar  changes,  but  in  large  measure  they  seem  to  be 

2*  Talbert,  quoted  by  I.  Bowman:  Forest  Physiography,  1911,  p.  63. 
25  J.  Barrell :   Ehythms  and  the  Measurement  of  Geologic  Time;   Bull. 
Geol.  Soc.  Am.,  Vol.  28,  1917,  pp.  745-904. 


214  CLIMATIC  CHANGES 

explained  by  variations  in  the  extent,  distribution,  and 
altitude  of  the  lands.  Such  variations  appear  to  be  the 
inevitable  result  of  the  earth's  contraction. 


CHAPTER  XII 


POST-GLACIAL  CRUSTAL  MOVEMENTS  AND 
CLIMATIC  CHANGES 

A  N  interesting  practical  application  of  some  of  the 
i\  preceding  generalizations  is  found  in  an  attempt 
7~%  by  C.  E.  P.  Brooks1  to  interpret  post-glacial 
climatic  changes  almost  entirely  in  terms  of  crustal  move- 
ment. We  believe  that  he  carries  the  matter  much  too  far, 
but  his  discussion  is  worthy  of  rather  full  recapitulation, 
not  only  for  its  theoretical  value  but  because  it  gives  a 
good  summary  of  post-glacial  changes.  His  climatic  table 
for  northwest  Europe  as  reprinted  from  the  annual  re- 
port of  the  Smithsonian  Institution  for  1917,  p.  366,  is 
as  follows : 


Phase 

1.  The  Last  Great  Glacia- 
tion. 
2.  The     Eetreat     of     the 
Glaciers. 
3.  The  Continental  Phase. 
4.  The  Maritime  Phase. 
5.  The  Later  Forest  Phase. 
6.  The  Peat-Bog  Phase. 
7.  The  Eecent  Phase. 

Climate 

Arctic  climate. 
Severe    continental 
climate. 
Continental  climate. 
Warm  and  moist. 
Warm  and  dry. 
Cooler  and  moister. 
Becoming  drier. 

Date 
30,000-18,000  B.  C. 

18,000-6000  B.  C. 
6000-4000  B.  C. 
4000-3000  B.  C. 
3000-1800  B.  C. 
1800  B.  C.-300  A.  D. 
300  A.  D.- 

Brooks  bases  his  chronology  largely  on  De   Geer's 
measurements   of  the   annual   layers   of  clay  in   lake 

i  C.  E.  P.  Brooks :  The  Evolution  of  Climate  in  Northwest  Europe.  Quart. 
Jour.  Koyal  Meteorol.  Soc.,  Vol.  47,  1921,  pp.  173-194. 


216  CLIMATIC  CHANGES 

bottoms  but  makes  much  use  of  other  evidence.  Accord- 
ing to  Brooks  the  last  glacial  epoch  lasted  roughly  from 
30,000  to  18,000  B.  C.,  but  this  includes  a  slight  ameliora- 
tion of  climate  followed  by  a  readvance  of  the  ice,  known 
as  the  Buhl  stage.  During  the  time  of  maximum  glacia- 
tion  the  British  Isles  stood  twenty  or  thirty  feet  higher 
than  now  and  Scandinavia  was  "considerably"  more 
elevated.  The  author  believes  that  this  caused  a  fall  of 
1°C.  in  the  temperature  of  the  British  Isles  and  of  2°C. 
in  Scandinavia.  By  an  ingenious  though  not  wholly  con- 
vincing method  of  calculation  he  concludes  that  this 
lowering  of  temperature,  aided  by  an  increase  in  the  area 
of  the  lands,  sufficed  to  start  an  ice  sheet  in  Scandinavia. 
The  relatively  small  area  of  ice  cooled  the  air  and  gave 
rise  to  an  area  of  high  barometric  pressure.  This  in  turn 
is  supposed  to  have  caused  further  expansion  of  the  ice 
and  to  have  led  to  full-fledged  glaciation. 

About  18,000  B.  C.  the  retreat  of  the  ice  began  in  good 
earnest.  Even  though  no  evidence  has  yet  been  found, 
Brooks  believes  there  must  have  been  a  change  in  the  dis- 
tribution of  land  and  sea  to  account  for  the  diminution  of 
the  ice.  The  ensuing  millenniums  formed  the  Magdale- 
nian  period  in  human  history,  the  last  stage  of  the  Paleo- 
lithic, when  man  lived  in  caves  and  reindeer  were  abun- 
dant in  central  Europe.2  At  first  the  ice  retreated  very 
slowly  and  there  were  periods  when  for  scores  of  years 
the  ice  edge  remained  stationary  or  even  readvanced. 
About  10,000  B.  C.  the  edge  of  the  ice  lay  along  the 
southern  coast  of  Sweden.  During  the  next  2000  years  it 
withdrew  more  rapidly  to  about  59°N.  Then  came  the 
Fennoscandian  pause,  or  Grschnitz  stage,  when  for  about 

2H.  F.  Osborn:  Men  of  the  Old  Stone  Age,  N.  Y.,  1915;  J.  M.  Tyler: 
The  New  Stone  Age  in  Northwestern  Europe,  N.  Y.,  1920. 


POST-GLACIAL  CRUSTAL  MOVEMENTS        217 

200  years  the  ice  edge  remained  in  one  position,  forming 
a  great  moraine.  Brooks  suggests  that  this  pause  about 
8000  B.  C.  was  due  to  the  closing  of  the  connection  be- 
tween the  Atlantic  Ocean  and  the  Baltic  Sea  and  the 
synchronous  opening  of  a  connection  between  the  Baltic 
and  the  White  Seas,  whereby  cold  Arctic  waters  replaced 
the  warmer  Atlantic  waters.  He  notes,  however,  that 
about  7500  B.  C.  the  obliquity  of  the  ecliptic  was  probably 
nearly  1°  greater  than  at  present.  This  he  calculates  to 
have  caused  the  climate  of  Germany  and  Sweden  to  be 
1°F.  colder  than  at  present  in  winter  and  1°F.  warmer  in 
summer. 

The  next  climatic  stage  was  marked  by  a  rise  of  tem- 
perature till  about  6000  B.  C.  During  this  period  the  ice 
at  first  retreated,  presumably  because  the  climate  was 
ameliorating,  although  no  cause  of  such  amelioration  is 
assigned.  At  length  the  ice  lay  far  enough  north  to  allow 
a  connection  between  the  Baltic  and  the  Atlantic  by  way 
of  Lakes  Wener  and  Wetter  in  southern  Sweden.  This  is 
supposed  to  have  warmed  the  Baltic  Sea  and  to  have 
caused  the  climate  to  become  distinctly  milder.  Next  the 
land  rose  once  more  so  that  the  Baltic  was  separated 
from  the  Atlantic  and  was  converted  into  the  Ancylus 
lake  of  fresh  water.  The  southwest  Baltic  region  then 
stood  400  feet  higher  than  now.  The  result  was  the  Daun 
stage,  about  5000  B.  C.,  when  the  ice  halted  or  perhaps 
readvanced  a  little,  its  front  being  then  near  Eagunda 
in  about  latitude  63°.  Why  such  an  elevation  did  not 
cause  renewed  glaciation  instead  of  merely  the  slight 
Daun  pause,  Brooks  does  not  explain,  although  his  calcu- 
lations as  to  the  effect  of  a  slight  elevation  of  the  land 
during  the  main  period  of  glaciation  from  30,000  to 
18,000  B.  C.  would  seem  to  demand  a  marked  readvance. 


218  CLIMATIC  CHANGES 

After  5000  B.  C.  there  ensued  a  period  when  the  cli- 
mate, although  still  distinctly  continental,  was  relatively 
mild.  The  winters,  to  be  sure,  were  still  cold  but  the 
summers  were  increasingly  warm.  In  Sweden,  for  ex- 
ample, the  types  of  vegetation  indicate  that  the  summer 
temperature  was  7°F.  higher  than  now.  Storms,  Brooks 
assumes,  were  comparatively  rare  except  on  the  outer 
fringe  of  Great  Britain.  There  they  were  sufficiently 
abundant  so  that  in  the  Northwest  they  gave  rise  to  the 
first  Peat-Bog  period,  during  which  swamps  replaced 
forests  of  birch  and  pine.  Southern  and  eastern  England, 
however,  probably  had  a  dry  continental  climate.  Even 
in  northwest  Norway  storms  were  rare  as  is  indicated  by 
remains  of  forests  on  islands  now  barren  because  of  the 
strong  winds  and  fierce  storms.  Farther  east  most  parts 
of  central  and  northern  Europe  were  relatively  dry.  This 
was  the  early  Neolithic  period  when  man  advanced  from 
the  use  of  unpolished  to  polished  stone  implements. 

Not  far  from  4000  B.  C.  the  period  of  continental  cli- 
mate was  replaced  by  a  comparatively  moist  maritime 
climate.  Brooks  believes  that  this  was  because  sub- 
mergence opened  the  mouth  of  the  Baltic  and  caused  the 
fresh  Ancylus  lake  to  give  place  to  the  so-called  Litorina 
sea.  The  temperature  in  Sweden  averaged  about  3°F. 
higher  than  at  present  and  in  southwestern  Norway  2°. 
More  important  than  this  was  the  small  annual  range  of 
temperature  due  to  the  fact  that  the  summers  were  cool 
while  the  winters  were  mild.  Because  of  the  presence  of 
a  large  expanse  of  water  in  the  Baltic  region,  storms,  as 
our  author  states,  then  crossed  Great  Britain  and  fol- 
lowed the  Baltic  depression,  carrying  the  moisture  far 
inland.  In  spite  of  the  additional  moisture  thus  available 
the  snow  line  in  southern  Norway  was  higher  than  now. 

At  this  point  Brooks  turns  to  other  parts  of  the  world. 


POST-GLACIAL  CRUSTAL  MOVEMENTS        219 

He  states  that  not  far  from  4000  B.  C.,  a  submergence 
of  the  lands,  rarely  amounting  to  more  than  twenty-five 
feet,  took  place  not  only  in  the  Baltic  region  but  in  Ire- 
land, Iceland,  Spitzbergen,  and  other  parts  of  the  Arctic 
Ocean,  as  well  as  in  the  White  Sea,  Greenland,  and  the 
eastern  part  of  North  America.  Evidences  of  a  mild  cli- 
mate are  found  in  all  those  places.  Similar  evidence  of  a 
mild  warm  climate  is  found  in  East  Africa,  East  Aus- 
tralia, Tierra  del  Fuego,  and  Antarctica.  The  dates  are 
not  established  with  certainty  but  they  at  least  fall  in  the 
period  immediately  preceding  the  present  epoch.  In  ex- 
planation of  these  conditions  Brooks  assumes  a  universal 
change  of  sea  level.  He  suggests  with  some  hesitation 
that  this  may  have  been  due  to  one  of  Pettersson 's 
periods  of  maximum  '  *  tide-generating  force. ' '  According 
to  Pettersson  the  varying  positions  of  the  moon,  earth, 
and  sun  cause  the  tides  to  vary  in  cycles  of  about  9,  90, 
and  1800  years,  though  the  length  of  the  periods  is  not 
constant.  When  tides  are  high  there  is  great  movement 
of  ocean  waters  and  hence  a  great  mixture  of  the  water 
at  different  latitudes.  This  is  supposed  to  cause  an 
amelioration  of  climate.  The  periods  of  maximum  and 
minimum  tide-generating  force  are  as  follows : 

Maxima  3500  B.  C. 2100  B.  C. 350  B.  C. A.  D.  1434 

Minima  2800  B.  C. 1200  B.  C. A.  D.  530 

Brooks  thinks  that  the  big  trees  in  California  and  the 
Norse  sagas  and  Germanic  myths  indicate  a  rough  agree- 
ment of  climatic  phenomena  with  Pettersson 's  last  three 
dates,  while  the  mild  climate  of  4000  B.  C.  may  really 
belong  to  3500  B.  C.  He  gives  no  evidence  confirming 
Pettersson 's  view  at  the  other  three  dates. 

To  return  to  Brooks '  sketch  of  the  relation  of  climatic 
pulsations  to  the  altitude  of  the  lands,  by  3000  B.  C.,  that 


220  CLIMATIC  CHANGES 

is,  toward  the  close  of  the  Neolithic  period,  further  eleva- 
tion is  supposed  to  have  taken  place  over  the  central 
latitudes  of  western  Europe.  Southern  Britain,  which  had 
remained  constantly  above  its  present  level  ever  since 
30,000  B.  C.,  was  perhaps  ninety  feet  higher  than  now. 
Ireland  was  somewhat  enlarged  by  elevation,  the  Straits 
of  Dover  were  almost  closed,  and  parts  of  the  present 
North  Sea  were  land.  To  these  conditions  Brooks  ascribes 
the  prevalence  of  a  dry  continental  climate.  The  storms 
shifted  northward  once  more,  the  winds  were  mild,  as 
seems  to  be  proved  by  remains  of  trees  in  exposed  places ; 
and  forests  replaced  fields  of  peat  and  heath  in  Britain 
and  Germany.  The  summers  were  perhaps  warmer  than 
now  but  the  winters  were  severe.  The  relatively  dry  cli- 
mate prevailed  as  far  west  as  Ireland.  For  example,  in 
Drumkelin  Bog  in  Donegal  County  a  corded  oak  road  and 
a  two-story  log  cabin  appear  to  belong  to  this  time.  Four- 
teen feet  of  bog  lie  below  the  floor  and  twenty-six  above. 
This  period,  perhaps  3000-2000  B.  C.,  was  the  legendary 
heroic  age  of  Ireland  when  "the  vigour  of  the  Irish 
reached  a  level  not  since  attained."  This,  as  Brooks 
points  out,  may  have  been  a  result  of  the  relatively  dry 
climate,  for  today  the  extreme  moisture  of  Ireland  seems 
to  be  a  distinct  handicap.  In  Scandinavia,  civilization,  or 
at  least  the  stage  of  relative  progress,  was  also  high  at 
this  time. 

By  1600  B.  C.  the  land  had  assumed  nearly  its  pres- 
ent level  in  the  British  Isles  and  the  southern  Baltic 
region,  while  northern  Scandinavia  still  stood  lower  than 
now.  The  climate  of  Britain  and  Germany  was  so  humid 
that  there  was  an  extensive  formation  of  peat  even  on 
high  ground  not  before  covered.  This  moist  stage  seems 
to  have  lasted  almost  to  the  time  of  Christ,  and  may  have 
been  the  reason  why  the  Eomans  described  Britain  as 


POST-GLACIAL  CRUSTAL  MOVEMENTS        221 

peculiarly  wet  and  damp.  At  this  point  Brooks  again  de- 
parts from  northwest  Europe  to  a  wider  field : 

It  is  possible  that  we  have  to  attribute  this  damp  period  in 
Northwest  Europe  to  some  more  general  cause,  for  Ellsworth 
Huntington's  curves  of  tree-growth  in  California  and  climate 
in  Western  Asia  both  show  moister  conditions  from  about 
1000  B.  C.  to  A.  D.  200,  and  the  same  author  believes  that  the 
Mediterranean  lands  had  a  heavier  rainfall  about  500  B.  C.  to 
A.  D.  200.  It  seems  that  the  phase  was  marked  by  a  general  in- 
crease of  the  storminess  of  the  temperate  regions  of  the  northern 
hemisphere  at  least,  with  a  maximum  between  Ireland  and  North 
Germany,  indicating  probably  that  the  Baltic  again  became  the 
favourite  track  of  depressions  from  the  Atlantic. 

Brooks  ends  his  paper  with  a  brief  resume  of  glacial 
changes  in  North  America,  but  as  the  means  of  dating 
events  are  unreliable  the  degree  of  synchronism  with 
Europe  is  not  clear.  He  sums  up  his  conclusions  as 
follows : 

On  the  whole  it  appears  that  though  there  is  a  general  simi- 
larity in  the  climatic  history  of  the  two  sides  of  the  North 
Atlantic,  the  changes  are  not  really  contemporaneous,  and  such 
relationship  as  appears  is  due  mainly  to  the  natural  similarity 
in  the  geographical  history  of  two  regions  both  recovering  from 
an  Ice  Age,  and  only  very  partially  to  world-wide  pulsations 
of  climate.  Additional  evidence  on  this  head  will  be  available 
when  Baron  de  Geer  publishes  the  results  of  his  recent  investiga- 
tions of  the  seasonal  glacial  clays  of  North  America,  especially 
if,  as  he  hopes,  he  is  able  to  correlate  the  banding  of  these  clays 
with  the  growth-rings  of  the  big  trees. 

"When  we  turn  to  the  northwest  of  North  America,  this  is 
brought  out  very  markedly.  For  in  Yukon  and  Alaska  the  Ice 
Age  was  a  very  mild  affair  compared  with  its  severity  in  eastern 
America  and  Scandinavia.  As  the  land  had  not  a  heavy  ice-load 
to  recover  from,  there  were  no  complicated  geographical 


222  CLIMATIC  CHANGES 

changes.  Also,  there  were  no  fluctuations  of  climate,  but  simply 
a  gradual  passage  to  present  conditions.  The  latter  circumstance 
especially  seems  to  show  that  the  emphasis  laid  on  geographical 
rather  than  astronomical  factors  of  great  climatic  changes  is 
not  misplaced. 

Brooks '  painstaking  discussion  of  post-glacial  climatic 
changes  is  of  great  value  because  of  the  large  body  of 
material  which  he  has  so  carefully  wrought  together.  His 
strong  belief  in  the  importance  of  changes  in  the  level  of 
the  lands  deserves  serious  consideration.  It  is  difficult, 
however,  to  accept  his  final  conclusion  that  such  changes 
are  the  main  factors  in  recent  climatic  changes.  It  is  al- 
most impossible,  for  example,  to  believe  that  movements 
of  the  land  could  produce  almost  the  same  series  of 
climatic  changes  in  Europe,  Central  Asia,  the  western 
and  eastern  parts  of  North  America,  and  the  southern 
hemisphere.  Yet  such  changes  appear  to  have  occurred 
during  and  since  the  glacial  period.  Again  there  is  no 
evidence  whatever  that  movements  of  the  land  have  any- 
thing to  do  with  the  historic  cycles  of  climate  or  with  the 
cycles  of  weather  in  our  own  day,  which  seem  to  be  the 
same  as  glacial  cycles  on  a  small  scale.  Also,  as  Dr. 
Simpson  points  out  in  discussing  Brooks'  paper,  there 
appears  "no  solution  along  these  lines  of  the  problem 
connected  with  rich  vegetation  in  both  polar  circles  and 
the  ice-age  which  produced  the  ice-sheet  at  sea-level  in 
Northern  India. ' '  Nevertheless,  we  may  well  believe  that 
Brooks  is  right  in  holding  that  changes  in  the  relative 
level  and  relative  area  of  land  and  sea  have  had  im- 
portant local  effects.  "While  they  are  only  one  of  the 
factors  involved  in  climatic  changes,  they  are  certainly 
one  that  must  constantly  be  kept  in  mind. 


CHAPTER  XIII 

THE  CHANGING  COMPOSITION  OF  OCEANS  AND 
ATMOSPHERE 

HAVING  discussed  the  climatic  effect  of  move- 
ments of  the  earth 's  crust  during  the  course  of 
geological  time,  we  are  now  ready  to  consider 
the  corresponding  effects  due  to  changes  in  the  movable 
envelopes — the  oceans  and  the  atmosphere.  Variations  in 
the  composition  of  sea  water  and  of  air,  and  in  the 
amount  of  air  must  almost  certainly  have  occurred,  and 
must  have  produced  at  least  slight  climatic  consequences. 
It  should  be  pointed  out  at  once  that  such  variations 
appear  to  be  far  less  important  climatically  than  do 
movements  of  the  earth's  crust  and  changes  in  the  activ- 
ity of  the  sun.  Moreover,  in  most  cases,  they  are  not 
reversible  as  are  the  crustal  and  solar  phenomena.  Hence, 
while  most  of  them  appear  to  have  been  unimportant  so 
far  as  climatic  oscillations  and  fluctuations  are  concerned, 
they  seemingly  have  aided  in  producing  the  slight  secular 
progression  to  which  we  have  so  often  referred. 

There  is  general  agreement  among  geologists  that  the 
ocean  has  become  increasingly  saline  throughout  the 
ages.  Indeed,  calculations  of  the  rate  of  accumulation  of 
salt  have  been  a  favorite  method  of  arriving  at  estimates 
of  the  age  of  the  ocean,  and  hence  of  the  earliest  marine 
sediments.  So  far  as  known,  however,  no  geologist  or 
climatologist  has  discussed  the  probable  climatic  effects 


224  CLIMATIC  CHANGES 

of  increased  salinity.  Yet  it  seems  clear  that  an  increase 
in  salinity  must  have  a  slight  effect  upon  climate. 

Salinity  affects  climate  in  four  ways:  (1)  It  appre- 
ciably influences  the  rate  of  evaporation;  (2)  it  alters 
the  freezing  point;  (3)  it  produces  certain  indirect 
effects  through  changes  in  the  absorption  of  carbon 
dioxide;  and  (4)  it  has  an  effect  on  oceanic  circulation. 

(1)  According   to    the    experiments    of   Mazelle    and 
Okada,  as  reported  by  Kriimmel,1  evaporation  from  ordi- 
nary sea  water  is  from  9  to  30  per  cent  less  rapid  than 
from  fresh  water  under  similar  conditions.  The  varia- 
tion from  9  to  30  per  cent  found  in  the  experiments  de- 
pends, perhaps,  upon  the  wind  velocity.  When  salt  water 
is  stagnant,  rapid  evaporation  tends  to  result  in  the 
development  of  a  film  of  salt  on  the  top  of  the  water, 
especially  where  it  is  sheltered  from  the  wind.  Such  a  film 
necessarily  reduces  evaporation.  Hence  the  relatively 
low  salinity  of  the  oceans  in  the  past  probably  had  a 
tendency  to  increase  the  amount  of  water  vapor  in  the 
air.  Even  a  little  water  vapor  augments  slightly  the 
blanketing  effect  of  the  air  and  to  that  extent  diminishes 
the  diurnal  and  seasonal  range  of  temperature  and  the 
contrast  from  zone  to  zone. 

(2)  Increased  salinity  means  a  lower  freezing  tem- 
perature of  the  oceans  and  hence  would  have  an  effect 
during  cold  periods  such  as  the  present  and  the  Pleisto- 
cene ice  age.  It  would  not,  however,  be  of  importance 
during  the   long  warm   periods   which   form   most   of 
geologic  time.  A  salinity  of  about  3.5  per  cent  at  present 
lowers  the  freezing  point  of  the  ocean  roughly  2°C.  below 
that  of  fresh  water.  If  the  ocean  were  fresh  and  our 
winters  as  cold  as  now,  all  the  harbors  of  New  England 
and  the  Middle  Atlantic  States  would  be  icebound.  The 

i  Encyclopedia  Britannica,  llth  edition:  article  "Ocean." 


OCEANS  AND  ATMOSPHERE  225 

Baltic  Sea  would  also  be  frozen  each  winter,  and  even 
the  eastern  harbors  of  the  British  Isles  would  be  fre- 
quently locked  in  ice.  At  high  latitudes  the  area  of  per- 
manently frozen  oceans  would  be  much  enlarged.  The 
effect  of  such  a  condition  upon  marine  life  in  high  lati- 
tudes would  be  like  that  of  a  change  to  a  warmer  climate. 
It  would  protect  the  life  on  the  continental  shelf  from  the 
severe  battering  of  winter  storms.  It  would  also  lessen 
the  severity  of  the  winter  temperature  in  the  water  for 
when  water  freezes  it  gives  up  much  latent  heat, — eighty 
calories  per  cubic  centimeter.  Part  oJJ.this  raises  the 
temperature  of  the  underlying  water.  ,  f 

The  expansion  of  the  ice  near  northern  shores  would 
influence  the  life  of  the  lands  quite  differently  from  that 
of  the  oceans.  It  would  act  like  an  addition  of  land  to  the 
continents  and  would,  therefore,  increase  the  atmospheric 
contrasts  from  zone  to  zone  and  from  continental  interior 
to  ocean.  In  summer  the  ice  upon  the  sea  would  tend  to 
keep  the  coastal  lands  cool,  very  much  as  happens  now 
near  the  Arctic  Ocean,  where  the  ice  floes  have  a  great 
effect  through  their  reflection  of  light  and  their  absorp- 
tion of  heat  in  melting.  In  winter  the  virtual  enlargement 
of  the  continents  by  the  addition  of  an  ice  fringe  would 
decrease  the  snowfall  upon  the  lands.  Still  more  im- 
portant would  be  the  effect  in  intensifying  the  anti- 
cyclonic  conditions  which  normally  prevail  in  winter  not 
only  over  continents  but  over  ice-covered  oceans.  Hence 
the  outblowing  cold  winds  would  be  strengthened.2  The 
net  effect  of  all  these  conditions  would  apparently  be  a 
diminution  of  snowfall  in  high  latitudes  upon  the  lands 
even  though  the  summer  snowfall  upon  the  ocean  and  the 

2  C.  E.  P.  Brooks :  The  Meteorological  Conditions  of  an  Ice  Sheet  and 
Their  Bearing  on  the  Desiccation  of  the  Globe ;  Quart.  Jour.  Eoyal  Meteorol. 
Soc.,  Vol.  40,  1914,  pp.  53-70. 


226  CLIMATIC  CHANGES 

coasts  may  have  increased.  This  condition  may  have  been 
one  reason  why  widespread  glaciation  does  not  appear  to 
have  prevailed  in  high  latitudes  during  the  Proterozoic 
and  Permian  glaciations,  even  though  it  occurred  farther 
south.  If  the  ocean  during  those  early  glacial  epochs 
were  ice-covered  down  to  middle  latitudes,  a  lack  of  ex- 
tensive glaciation  in  high  latitudes  would  be  no  more 
surprising  than  is  the  lack  of  Pleistocene  glaciation  in 
the  northern  parts  of  Alaska  and  Asia.  Great  ice  sheets 
are  impossible  without  a  large  supply  of  moisture. 

(3)  Among  the  indirect  effects  of  salinity  one  of  the 
chief  appears  to  be  that  the  low  salinity  of  the  water  in 
the  past  and  the  greater  ease  with  which  it  froze  presum- 
ably allowed  the  temperature  of  the  entire  ocean  to  be 
slightly  higher  than  now.  This  is  because  ice  serves  as  a 
blanket  and  hinders  the  radiation  of  heat  from  the  under- 
lying water.  The  temperature  of  the  ocean  has  a  climatic 
significance  not  only  directly,  but  indirectly  through  its 
influence  on  the  amount  of  carbon  dioxide  held  by  the 
oceans.  A  change  of  even  1°C.  from  the  present  mean 
temperature  of  2°C.  would  alter  the  ability  of  the  entire 
ocean  to  absorb  carbon  dioxide  by  about  4  per  cent.  This, 
according  to  F.  W.  Clarke,3  is  because  the  oceans  contain 
from  eighteen  to  twenty-seven  times  as  much  carbon 
dioxide  as  the  air  when  only  the  free  carbon  dioxide  is 
considered,  and  about  seventy  times  as  much  according 
to  Johnson  and  Williamson*  when  the  partially  combined 
carbon  dioxide  is  also  considered.  Moreover,  the  capacity 
of  water  for  carbon  dioxide  varies  sharply  with  the  tem- 
perature.5 Hence  a  rise  in  temperature  of  only  1°C. 
would  theoretically  cause  the  oceans  to  give  up  from  30 

s  Data  of  Geochemistry,  Fourth  Ed.,  1920;  Bull.  No.  695,  U.  S.  Geol. 
Survey. 

*  Quoted  by  Schuchert  in  The  Evolution  of  the  Earth. 

s  Smithsonian  Physical  Tables,  Sixth  Eevision,  1914,  p.  142. 


OCEANS  AND  ATMOSPHERE  227 

to  280  times  as  much  carbon  dioxide  as  the  air  now  holds. 
This,  however,  is  on  the  unfounded  assumption  that  the 
oceans  are  completely  saturated.  The  important  point  is 
merely  that  a  slight  change  in  ocean  temperature  would 
cause  a  disproportionately  large  change  in  the  amount 
of  carbon  dioxide  in  the  air  with  all  that  this  implies  in 
respect  to  blanketing  the  earth,  and  thus  altering  tem- 
perature. 

(4)  Another  and  perhaps  the  most  important  effect  of 
salinity  upon  climate  depends  upon  the  rapidity  of  the 
deep-sea  circulation.  The  circulation  is  induced  by  differ- 
ences of  temperature,  but  its  speed  is  affected  at  least 
slightly  by  salinity.  The  vertical  circulation  is  now  domi- 
nated by  cold  water  from  subpolar  latitudes.  Except  in 
closed  seas  like  the  Mediterranean  the  lower  portions 
of  the  ocean  are  near  the  freezing  point.  This  is  because 
cold  water  sinks  in  high  latitudes  by  reason  of  its  su- 
perior density,  and  then  '  *  creeps ' '  to  low  latitudes.  There 
it  finally  rises  and  replaces  either  the  water  driven  pole- 
ward by  the  winds,  or  that  which  has  evaporated  from 
the  surface.6 

During  past  ages,  when  the  sea  water  was  less  salty, 
the  circulation  was  presumably  more  rapid  than  now. 
This  was  because,  in  tropical  regions,  the  rise  of  cold 

6  Chamberlin,  in  a  very  suggestive  article  "On  a  possible  reversal  of 
oceanic  circulation"  (Jour,  of  Geol.,  Vol.  14,  pp.  363-373,  1906),  discusses 
the  probable  climatic  consequences  of  a  reversal  in  the  direction  of  deep- 
sea  circulation.  It  is  not  wholly  beyond  the  bounds  of  possibility  that 
in  the  course  of  ages  the  increasing  drainage  of  salt  from  the  lands  not 
only  by  nature  but  by  man's  activities  in  agriculture  and  drainage,  may 
ultimately  cause  such  a  reversal  by  increasing  the  ocean's  salinity  until  the 
more  saline  tropical  portion  is  heavier  than  the  cooler  but  fresher  subpolar 
waters.  If  that  should  happen,  Greenland,  Antarctica,  and  the  northern 
shores  of  America  and  Asia  would  be  warmed  by  the  tropical  heat  which 
had  been  transferred  poleward  beneath  the  surface  of  the  ocean,  without 
loss  en  route.  Subpolar  regions,  under  such  a  condition  of  reversed  deep-sea 
circulation,  might  have  a  mild  climate.  Indeed,  they  might  be  among  the 
world's  most  favorable  regions  climatically. 


228  CLIMATIC  CHANGES 

water  is  hindered  by  the  sinking  of  warm  surface  water 
which  is  relatively  dense  because  evaporation  has  re- 
moved part  of  the  water  and  caused  an  accumulation  of 
salt.  According  to  Kriimmel  and  Mill,7  the  surface  salin- 
ity of  the  subtropical  belt  of  the  North  Atlantic  commonly 
exceeds  3.7  per  cent  and  sometimes  reaches  3.77  per  cent, 
whereas  the  underlying  waters  have  a  salinity  of  less 
than  3.5  per  cent  and  locally  as  little  as  3.44  per  cent. 
The  other  oceans  are  slightly  less  saline  than  the  North 
Atlantic  at  all  depths,  but  the  vertical  salinity  gradients 
along  the  tropics  are  similar.  According  to  the  Smith- 
sonian Physical  Tables,  the  difference  in  salinity  between 
the  surface  water  and  that  lying  below  is  equivalent  to 
a  difference  of  .003  in  density,  where  the  density  of  fresh 
water  is  taken  as  1.000.  Since  the  decrease  in  density  pro- 
duced by  warming  water  from  the  temperature  of  its 
greatest  density  (4°C.)  to  the  highest  temperatures 
which  ever  prevail  in  the  ocean  (30°C.  or  86°F.)  is  only 
.004,  the  more  saline  surface  waters  of  the  dry  tropics 
are  at  most  times  almost  as  dense  as  the  less  saline  but 
colder  waters  beneath  the  surface,  which  have  come  from 
higher  latitudes.  During  days  of  especially  great  evapo- 
ration, however,  the  most  saline  portions  of  the  surface 
waters  in  the  dry  tropics  are  denser  than  the  underlying 
waters  and  therefore  sink,  and  produce  a  temporary  local 
stagnation  in  the  general  circulation.  Such  a  sinking  of 
the  warm  surface  waters  is  reported  by  Kriimmel,  who 
detected  it  by  means  of  the  rise  in  temperature  which  it 
produces  at  considerable  depths.  If  such  a  hindrance  to 
the  circulation  did  not  exist,  the  velocity  of  the  deep-sea 
movements  would  be  greater. 

If  in  earlier  times  a  more  rapid  circulation  occurred, 
low  latitudes  must  have  been  cooled  more  than  now  by 

7  Encyclopaedia  Britanniea :  article  ' '  Ocean. ' ' 


OCEANS  AND  ATMOSPHERE 


229 


the  rise  of  cold  waters.  At  the  same  time  higher  latitudes 
were  presumably  warmed  by  a  greater  flow  of  warm 
water  from  tropical  regions  because  less  of  the  surface 
heat  sank  in  low  latitudes.  Such  conditions  would  tend  to 
lessen  the  climatic  contrast  between  the  different  lati- 
tudes. Hence,  in  so  far  as  the  rate  of  deep-sea  circulation 
depends  upon  salinity,  the  slowly  increasing  amount  of 
salt  in  the  oceans  must  have  tended  to  increase  the  con- 
trasts between  low  and  high  latitudes.  Thus  for  several 
reasons,  the  increase  of  salinity  during  geologic  history 
seems  to  deserve  a  place  among  the  minor  agencies  which 
help  to  explain  the  apparent  tendency  toward  a  secular 
progression  of  climate  in  the  direction  of  greater  con- 
trasts between  tropical  and  subpolar  latitudes. 

Changes  in  the  composition  and  amount  of  the  atmos- 
phere have  presumably  had  a  climatic  importance  greater 
than  that  of  changes  in  the  salinity  of  the  oceans.  The 
atmospheric  changes  may  have  been  either  progressive 
or  cyclic,  or  both.  In  early  times,  according  to  the  nebular 
hypothesis,  the  atmosphere  was  much  more  dense  than 
now  and  contained  a  larger  percentage  of  certain  con- 
stituents, notably  carbon  dioxide  and  water.  The  plane- 
tesimal  hypothesis,  on  the  other  hand,  postulates  an  in- 
crease in  the  density  of  the  atmosphere,  for  according  to 
this  hypothesis  the  density  of  the  atmosphere  depends 
upon  the  power  of  the  earth  to  hold  gases,  and  this  power 
increases  as  the  earth  grows  bigger  with  the  infall  of 
material  from  without.8 

Whichever  hypothesis  may  be  correct,  it  seems  prob- 
able that  when  life  first  appeared  on  the  land  the  at- 
mosphere resembled  that  of  today  in  certain  fundamental 
respects.  It  contained  the  elements  essential  to  life,  and 

s  Chamberlin  and  Salisbury:  Geology,  Vol.  II,  pp.  1-132,  1906;  and  T.  C. 
Chamberlin :  The  Origin  of  the  Earth,  1916. 


230  CLIMATIC  CHANGES 

its  blanketing  effect  was  such  as  to  maintain  tempera- 
tures not  greatly  different  from  those  of  the  present.  The 
evidence  of  this  depends  largely  upon  the  narrow  limits 
of  temperature  within  which  the  activities  of  modern 
life  are  possible,  and  upon  the  cumulative  evidence  that 
ancient  life  was  essentially  similar  to  the  types  now 
living.  The  resemblance  between  some  of  the  oldest 
forms  and  those  of  today  is  striking.  For  example, 
according  to  Professor  Schuchert:9  "Many  of  the  living 
genera  of  forest  trees  had  their  origin  in  the  Cretaceous, 
and  the  giant  sequoias  of  California  go  back  to  the  Trias- 
sic,  while  Ginkgo  is  known  in  the  Permian.  Some  of  the 
fresh-water  molluscs  certainly  were  living  in  the  early 
periods  of  the  Mesozoic,  and  the  lung-fish  of  today 
(Ceratodus)  is  known  as  far  back  as  the  Triassic  and  is 
not  very  unlike  other  lung-fishes  of  the  Devonian.  The 
higher  vertebrates  and  insects,  on  the  other  hand,  are 
very  sensitive  to  their  environment,  and  therefore  do  not 
extend  back  generically  beyond  the  Cenozoic,  and  only  in 
a  few  instances  even  as  far  as  the  Oligocene.  Of  marine 
invertebrates  the  story  is  very  different,  for  it  is  well 
known  that  the  horseshoe  crab  (Limulus)  lived  in  the 
Upper  Jurassic,  and  Nautilus  in  the  Triassic,  with  forms 
in  the  Devonian  not  far  removed  from  this  genus.  Still 
longer-ranging  genera  occur  among  the  brachiopods,  for 
living  Lingula  and  Crania  have  specific  representatives 
as  far  back  as  the  early  Ordovician.  Among  living  fora- 
minifers,  Lagena,  Globigerina,  and  Nodosaria  are  known 
in  the  later  Cambrian  or  early  Ordovician.  In  the  Middle 
Cambrian  near  Field,  British  Columbia,  Walcott  has 
found  a  most  varied  array  of  invertebrates  among  which 
are  crustaceans  not  far  removed  from  living  forms. 
Zoologists  who  see  these  wonderful  fossils  are  at  once 

8  Personal  communication. 


OCEANS  AND  ATMOSPHERE  231 

struck  with  their  modernity  and  the  little  change  that  has 
taken  place  in  certain  stocks  since  that  far  remote  time. 
Back  of  the  Paleozoic,  little  can  be  said  of  life  from  the 
generic  standpoint,  since  so  few  fossils  have  been  re- 
covered, but  what  is  at  hand  suggests  that  the  marine 
environment  was  similar  to  that  of  today. ' ' 

At  present,  as  we  have  repeatedly  seen,  little  growth 
takes  place  either  among  animals  or  plants  at  tempera- 
tures below  0°C.  or  above  40°C.,  and  for  most  species 
the  limiting  temperatures  are  about  10°  and  30°.  The 
maintenance  of  so  narrow  a  scale  of  temperature  is  a 
function  of  the  atmosphere,  as  well  as  of  the  sun.  Without 
an  atmosphere,  the  temperature  by  day  would  mount 
fatally  wherever  the  sun  rides  high  in  the  sky.  By  night 
it  would  fall  everywhere  to  a  temperature  approaching 
absolute  zero,  that  is  — 273  °C.  Some  such  tempera- 
ture prevails  a  few  miles  above  the  earth's  surface, 
beyond  the  effective  atmosphere.  Indeed,  even  if  the 
atmosphere  were  almost  as  it  is  now,  but  only  lacked  one 
of  the  minor  constituents,  a  constituent  which  is  often 
actually  ignored  in  statements  of  the  composition  of  the 
air,  life  would  be  impossible.  Tyndall  concludes  that  if 
water  vapor  were  entirely  removed  from  the  atmosphere 
for  a  single  day  and  night,  all  life — except  that  which  is 
dormant  in  the  form  of  seeds,  eggs,  or  spores — would  be 
exterminated.  Part  would  be  killed  by  the  high  tempera- 
ture developed  by  day  when  the  sun  was  high,  and  part, 
by  the  cold  night. 

The  testimony  of  ancient  glaciation  as  to  the  slight 
difference  in  the  climate  and  therefore  in  the  atmos- 
phere of  early  and  late  geological  times  is  almost  as  clear 
as  that  of  life.  Just  as  life  proves  that  the  earth  can  never 
have  been  extremely  cold  during  hundreds  of  millions  of 
years,  so  glaciation  in  moderately  low  latitudes  near 


232  CLIMATIC  CHANGES 

the  dawn  of  earth  history  and  at  several  later  times, 
proves  that  the  earth  was  not  particularly  hot  even  in 
those  early  days.  The  gentle  progressive  change  of  climate 
which  is  recorded  in  the  rocks  appears  to  have  been  only 
in  slight  measure  a  change  in  the  mean  temperature  of 
the  earth  as  a  whole,  and  almost  entirely  a  change  in  the 
distribution  of  temperature  from  place  to  place  and 
season  to  season.  Hence  it  seems  probable  that  neither 
the  earth's  own  emission  jof  heat,  nor  the  supply  of  solar 
heat,  nor  the  power  of  the  atmosphere  to  retain  heat  can 
have  been  much  greater  a  few  hundred  million  years  ago 
than  now.  It  is  indeed  possible  that  these  three  factors 
may  have  varied  in  such  a  way  that  any  variation  in  one 
has  been  offset  by  variations  of  the  others  in  the  opposite 
direction.  This,  however,  is  so  highly  improbable  that  it 
seems  advisable  to  assume  that  all  three  have  remained 
relatively  constant.  This  conclusion  together  with  a 
realization  of  the  climatic  significance  of  carbon  dioxide 
has  forced  most  of  the  adherents  of  the  nebular  hypothe- 
sis to  abandon  their  assumption  that  carbon  dioxide,  the 
heaviest  gas  in  the  air,  was  very  abundant  until  taken 
out  by  coal-forming  plants  or  combined  with  the  calcium 
oxide  of  igneous  rocks  to  form  the  limestone  secreted 
by  animals.  In  the  same  way  the  presence  of  sun  cracks 
in  sedimentary  rocks  of  all  ages  suggests  that  the  air 
cannot  have  contained  vast  quantities  of  water  vapor 
such  as  have  been  assumed  by  Knowlton  and  others  in 
order  to  account  for  the  former  lack  of  sharp  climatic 
contrast  between  the  zones.  Such  a  large  amount  of  water 
vapor  would  almost  certainly  be  accompanied  by  well- 
nigh  universal  and  continual  cloudiness  so  that  there 
would  be  little  chance  for  the  pools  on  the  earth's  water- 
soaked  surface  to  dry  up.  Furthermore,  there  is  only  one 
way  in  which  such  cloudiness  could  be  maintained  and 


OCEANS  AND  ATMOSPHERE  233 

that  is  by  keeping  the  air  at  an  almost  constant  tempera- 
ture night  and  day.  This  would  require  that  the  chief 
source  of  warmth  be  the  interior  of  the  earth,  a  condition 
which  the  Proterozoic,  Permian,  and  other  widespread 
glaciations  seem  to  disprove. 

Thus  there  appears  to  be  strong  evidence  against  the 
radical  changes  in  the  atmosphere  which  are  sometimes 
postulated.  Yet  some  changes  must  have  taken  place,  and 
even  minor  changes  would  be  accompanied  by  some  sort 
of  climatic  effect.  The  changes  would  take  the  form  of 
either  an  increase  or  a  decrease  in  the  atmosphere  as  a 
whole,  or  in  its  constituent  elements.  The  chief  means  by 
which  the  atmosphere  has  increased  appear  to  be  as 
follows:  (a)  By  contributions  from  the  interior  of  the 
earth  via  volcanoes  and  springs  and  by  the  weathering  of 
igneous  rocks  with  the  consequent  release  of  their  en- 
closed gases;10  (b)  by  the  escape  of  some  of  the  abundant 
gases  which  the  ocean  holds  in  solution ;  (c)  by  the  arrival 
on  the  earth  of  gases  from  space,  either  enclosed  in 
meteors  or  as  free-flying  molecules;  (d)  by  the  release  of 
gases  from  organic  compounds  by  oxidation,  or  by  ex- 
halation from  animals  and  plants.  On  the  other  hand,  one 
or  another  of  the  constituents  of  the  atmosphere  has  pre- 
sumably decreased  (a)  by  being  locked  up  in  newly 
formed  rocks  or  organic  compounds;  (b)  by  being  dis- 
solved in  the  ocean;  (c)  by  the  escape  of  molecules  into 
space;  and  (d)  by  the  condensation  of  water  vapor. 

The  combined  effect  of  the  various  means  of  increase 
and  decrease  depends  partly  on  the  amount  of  each  con- 
stituent received  from  the  earth's  interior  or  from  space, 
and  partly  on  the  fact  that  the  agencies  which  tend  to 
deplete  the  atmosphere  are  highly  selective  in  their 

10  E.  T.  Chamberlin:  Gases  in  Rocks,  Carnegie  Inst.  of  Wash.,  No.  106, 
1908. 


234  CLIMATIC  CHANGES 

action.  Our  knowledge  of  how  large  a  quantity  of  new 
gases  the  air  has  received  is  very  scanty,  but  judging  by 
present  conditions  the  general  tendency  is  toward  a  slow 
increase  chiefly  because  of  meteorites,  volcanic  action, 
and  the  work  of  deep-seated  springs.  As  to  decrease,  the 
case  is  clearer.  This  is  because  the  chemically  active 
gases,  oxygen,  C02,  and  water  vapor,  tend  to  be  locked 
up  in  the  rocks,  while  the  chemically  inert  gases,  nitrogen 
and  argon,  show  almost  no  such  tendency.  Though  oxy- 
gen is  by  far  the  most  abundant  element  in  the  earth's 
crust,  making  up  more  than  50  per  cent  of  the  total,  it 
forms  only  about  one-fifth  of  the  air.  Nitrogen,  on  the 
other  hand,  is  very  rare  in  the  rocks,  but  makes  up  nearly 
four-fifths  of  the  air.  It  would,  therefore,  seem  probable 
that  throughout  the  earth's  history,  there  has  been  a 
progressive  increase  in  the  amount  of  atmospheric  nitro- 
gen, and  presumably  a  somewhat  corresponding  increase 
in  the  mass  of  the  air.  On  the  other  hand,  it  is  not  clear 
what  changes  have  occurred  in  the  amount  of  atmos- 
pheric oxygen.  It  may  have  increased  somewhat  or 
perhaps  even  notably.  Nevertheless,  because  of  the 
greater  increase  in  nitrogen,  it  may  form  no  greater  per- 
centage of  the  air  now  than  in  the  distant  past. 

As  to  the  absolute  amounts  of  oxygen,  Barrell11 
thought  that  atmospheric  oxygen  began  to  be  present 
only  after  plants  had  appeared.  It  will  be  recalled  that 
plants  absorb  carbon  dioxide  and  separate  the  carbon 
from  the  oxygen,  using  the  carbon  in  their  tissues  and 
setting  free  the  oxygen.  As  evidence  of  a  paucity  of 
oxygen  in  the  air  in  early  Proterozoic  times,  Barrell 
cites  the  fact  that  the  sedimentary  rocks  of  that  remote 

11  J.  Barrell :  The  Origin  of  the  Earth,  in  Evolution  of  the  Earth  and 
Its  Inhabitants,  1918,  p.  44,  and  more  fully  in  an  unpublished  manuscript. 


OCEANS  AND  ATMOSPHERE  235 

time  commonly  are  somewhat  greyish  or  greenish-grey 
wackes,  or  other  types,  indicating  incomplete  oxidation. 
He  admits,  however,  that  the  stupendous  thicknesses  of 
red  sandstones,  quartzite,  and  hematitic  iron  ores  of  the 
later  Proterozoic  prove  that  by  that  date  there  was  an 
abundance  of  atmospheric  oxygen.  If  so,  the  change  from 
paucity  to  abundance  must  have  occurred  before  fossils 
were  numerous  enough  to  give  much  clue  to  climate. 
However,  Barrell's  evidence  as  to  a  former  paucity  of 
atmospheric  oxygen  is  not  altogether  convincing.  In  the 
first  place,  it  does  not  seem  justifiable  to  assume  that 
there  could  be  no  oxygen  until  plants  appeared  to  break 
down  the  carbon  dioxide,  for  some  oxygen  is  contributed 
by  volcanoes,12  and  lightning  decomposes  water  into  its 
elements.  Part  of  the  hydrogen  thus  set  free  escapes  into 
space,  for  the  earth's  gravitative  force  does  not  appear 
great  enough  to  hold  this  lightest  of  gases,  but  the  oxy- 
gen remains.  Thus  electrolysis  of  water  results  in  the 
accumulation  of  oxygen.  In  the  second  place,  there  is  no 
proof  that  the  ancient  greywackes  are  not  deoxidized 
sediments.  Light  colored  rock  formations  do  not  neces- 
sarily indicate  a  paucity  of  atmospheric  oxygen,  for  such 
rocks  are  abundant  even  in  recent  times.  For  example, 
the  Tertiary  formations  are  characteristically  light 
colored,  a  result,  however,  of  deoxidation.  Finally,  the 
fact  that  sedimentary  rocks,  irrespective  of  their  age, 
contain  an  average  of  about  1.5  per  cent  more  oxygen 
than  do  igneous  rocks,13  suggests  that  oxygen  was  pres- 
ent in  the  air  in  quantity  even  when  the  earliest  shales 
and  sandstones  were  formed,  for  atmospheric  oxygen 
seems  to  be  the  probable  source  of  the  extra  oxygen  they 

12  F.  W.  Clarke:  Data  of  Geochemistry,  Fourth  Ed.,  1920,  Bull.  No.  695, 
U.  S.  Geol.  Survey,  p.  256. 

is  F.  W.  Clarke:  loc.  cit.,  pp.  27-34  et  al 


236  CLIMATIC  CHANGES 

contain.  The  formation  of  these  particular  sedimentary 
rocks  by  weathering  of  igneous  rocks  involves  only  a 
little  carbon  dioxide  and  water.  Although  it  seems  prob- 
able that  oxygen  was  present  in  the  atmosphere  even  at 
the  beginning  of  the  geological  record,  it  may  have  been 
far  less  abundant  then  than  now.  It  may  have  been  re- 
moved from  the  atmosphere  by  animals  or  by  the  oxida- 
tion of  the  rocks  almost  as  rapidly  as  it  was  added  by 
volcanoes,  plants,  and  other  agencies.  ii\UU^~ 

After  this  chapter  was  in  type,  St.  John1*1  announced 
his  interesting  discovery  that  oxygen  is  apparently  lack- 
ing in  the  atmosphere  of  Venus.  He  considers  that  this 
proves  that  Venus  has  no  life.  Furthermore  he  concludes 
that  so  active  an  element  as  oxygen  cannot  be  abundant 
in  the  atmosphere  of  a  planet  unless  plants  continually 
supply  large  quantities  by  breaking  down  carbon  dioxide. 

But  even  if  the  earth  has  experienced  a  notable  in- 
crease in  atmospheric  oxygen  since  the  appearance  of 
life,  this  does  not  necessarily  involve  important  climatic 
changes  except  those  due  to  increased  atmospheric  den- 
sity. This  is  because  oxygen  has  very  little  effect  upon 
the  passage  of  light  or  heat,  being  transparent  to  all  but 
a  few  wave  lengths.  Those  absorbed  are  chiefly  in  the 
ultra  violet. 

The  distinct  possibility  that  oxygen  has  increased  in 
amount,  makes  it  the  more  likely  that  there  has  been  an 
increase  in  the  total  atmosphere,  for  the  oxygen  would 
supplement  the  increase  in  the  relatively  inert  nitrogen 
and  argon,  which  has  presumably  taken  place.  The  cli- 
matic effects  of  an  increase  in  the  atmosphere  include,  in 
the  first  place,  an  increased  scattering  of  light  as  it 
approaches  the  earth.  Nitrogen,  argon,  and  oxygen  all 

isa  Chas.  E.  St.  John :  Science  Service  Press  Reports  from  the  Mt.  Wilson 
Observatory,  May,  1922. 


OCEANS  AND  ATMOSPHERE  237 

scatter  the  short  waves  of  light  and  thus  interfere  with 
their  reaching  the  earth.  Abbot  and  Fowle,14  who  have 
carefully  studied  the  matter,  believe  that  at  present  the 
scattering  is  quantitatively  important  in  lessening  insola- 
tion. Hence  our  supposed  general  increase  in  the  volume 
of  the  air  during  part  of  geological  times  would  tend  to 
reduce  the  amount  of  solar  energy  reaching  the  earth's 
surface.  On  the  other  hand,  nitrogen  and  argon  do  not 
appear  to  absorb  the  long  wave  lengths  known  as  heat, 
and  oxygen  absorbs  so  little  as  to  be  almost  a  non- 
absorber.  Therefore  the  reduced  penetration  of  the  air 
by  solar  radiation  due  to  the  scattering  of  light  would 
apparently  not  be  neutralized  by  any  direct  increase  in 
the  blanketing  effect  of  the  atmosphere,  and  the  tempera- 
ture near  the  earth's  surface  would  be  slightly  lowered 
by  a  thicker  atmosphere.  This  would  diminish  the  amount 
of  water  vapor  which  would  be  held  in  the  air,  and 
thereby  lower  the  temperature  a  trifle  more. 

In  the  second  place,  the  higher  atmospheric  pressure 
which  would  result  from  the  addition  of  gases  to  the 
air  would  cause  a  lessening  of  the  rate  of  evaporation, 
for  that  rate  declines  as  pressure  increases.  Decreased 
evaporation  would  presumably  still  further  diminish  the 
vapor  content  of  the  atmosphere.  This  would  mean  a 
greater  daily  and  seasonal  range  of  temperature,  as  is 
very  obvious  when  we  compare  clear  weather  with  cloudy. 
Cloudy  nights  are  relatively  warm  while  clear  nights  are 
cool,  because  water  vapor  is  an  almost  perfect  absorber 
of  radiant  heat,  and  there  is  enough  of  it  in  the  air  on 
moist  nights  to  interfere  greatly  with  the  escape  of  the 
heat  accumulated  during  the  day.  Therefore,  if  atmos- 

i*  Abbot  and  Fowle :  Annals  Astrophysical  Observatory ;  Smiths.  Inst., 
Vol.  II,  1908,  p.  163. 

F.  E.  Fowle:  Atmospheric  Scattering  of  Light;  Misc.  Coll.  Smiths.  Inst., 
Vol.  69,  1918. 


238  CLIMATIC  CHANGES 

pheric  moisture  were  formerly  much  more  abundant 
than  now,  the  temperature  must  have  been  much  more 
uniform.  The  tendency  toward  climatic  severity  as  time 
went  on  would  be  still  further  increased  by  the  cooling 
which  would  result  from  the  increased  wind  velocity  dis- 
cussed below;  for  cooling  by  convection  increases  with 
the  velocity  of  the  wind,  as  does  cooling  by  conduction. 

Any  persistent  lowering  of  the  general  temperature  of 
the  air  would  affect  not  only  its  ability  to  hold  water 
vapor,  but  would  produce  a  lessening  in  the  amount  of 
atmospheric  carbon  dioxide,  for  the  colder  the  ocean 
becomes  the  more  carbon  dioxide  it  can  hold  in  solution. 
When  the  oceanic  temperature  falls,  part  of  the  atmos- 
pheric carbon  dioxide  is  dissolved  in  the  ocean.  This 
minor  constituent  of  the  air  is  important  because 
although  it  forms  only  0.003  per  cent  of  the  earth's  at- 
mosphere, Abbot  and  Fowle  's15  calculations  indicate  that 
it  absorbs  over  10  per  cent  of  the  heat  radiated  outward 
from  the  earth.  Hence  variations  in  the  amount  of  carbon 
dioxide  may  have  caused  an  appreciable  variation  in 
temperature  and  thus  in  other  climatic  conditions. 
Humphreys,  as  we  have  seen,  has  calculated  that  a 
doubling  of  the  carbon  dioxide  in  the  air  would  directly 
raise  the  earth's  temperature  to  the  extent  of  1.3°C.,  and 
a  halving  would  lower  it  a  like  amount.  The  indirect 
results  of  such  an  increase  or  decrease  might  be  greater 
than  the  direct  results,  for  the  change  in  temperature 
due  to  variations  in  carbon  dioxide  would  alter  the 
capacity  of  the  air  to  hold  moisture. 

Two  conditions  would  especially  help  in  this  respect; 
first,  changes  in  nocturnal  cooling,  and  second,  changes 
in  local  convection.  The  presence  of  carbon  dioxide  dimin- 
ishes nocturnal  cooling  because  it  absorbs  the  heat  radi- 

15  Abbot  and  Fowle:  loc.  cit.,  p.  172. 


OCEANS  AND  ATMOSPHERE  239 

ated  by  the  earth,  and  re-radiates  part  of  it  back  again. 
Hence  with  increased  carbon  dioxide  and  with  the 
consequent  warmer  nights  there  would  be  less  nocturnal 
condensation  of  water  vapor  to  form  dew  and  frost. 
Local  convection  is  influenced  by  carbon  dioxide  because 
this  gas  lessens  the  temperature  gradient.  In  general,  the 
less  the  gradient,  that  is,  the  less  the  contrast  between 
the  temperature  at  the  surface  and  higher  up,  the  less 
convection  takes  place.  This  is  illustrated  by  the  seasonal 
variation  in  convection.  In  summer,  when  the  gradient  is 
steepest,  convection  reaches  its  maximum.  It  will  be  re- 
called that  when  air  rises  it  is  cooled  by  expansion,  and 
if  it  ascends  far  the  moisture  is  soon  condensed  and 
precipitated.  Indeed,  local  convection  is  considered  by 
C.  P.  Day  to  be  the  chief  agency  which  keeps  the  lower 
air  from  being  continually  saturated  with  moisture.  The 
presence  of  carbon  dioxide  lessens  convection  because  it 
increases  the  absorption  of  heat  in  the  zone  above  the 
level  in  which  water  vapor  is  abundant,  thus  warming 
these  higher  layers.  The  lower  air  may  not  be  warmed 
correspondingly  by  an  increase  in  carbon  dioxide  if 
Abbot  and  Fowle  are  right  in  stating  that  near  the  earth 's 
surface  there  is  enough  water  vapor  to  absorb  practically 
all  the  wave  lengths  which  carbon  dioxide  is  capable  of 
absorbing.  Hence  carbon  dioxide  is  chiefly  effective  at 
heights  to  which  the  low  temperature  prevents  water 
vapor  from  ascending.  Carbon  dioxide  is  also  effective 
in  cold  winters  and  in  high  latitudes  when  even  the  lower 
air  is  too  cold  to  contain  much  water  vapor.  Moreover, 
carbon  dioxide,  by  altering  the  amount  of  atmospheric 
water  vapor,  exerts  an  indirect  as  well  as  a  direct  effect 
upon  temperature. 

Other  effects  of  the  increase  in  air  pressure  which  we 
are  here  assuming  during  at  least  the  early  part  of  geo- 


240  CLIMATIC  CHANGES 

logical  times  are  corresponding  changes  in  barometric 
contrasts,  in  the  strength  of  winds,  and  in  the  mass  of  air 
carried  by  the  winds  along  the  earth's  surface.  The  in- 
crease in  the  mass  of  the  air  would  reenforce  the  greater 
velocity  of  the  winds  in  their  action  as  eroding  and  trans- 
porting agencies.  Because  of  the  greater  weight  of  the 
air,  the  winds  would  be  capable  of  picking  up  more  dust 
and  of  carrying  it  farther  and  higher ;  while  the  increased 
atmospheric  friction  would  keep  it  aloft  a  longer  time. 
The  significance  of  dust  at  high  levels  and  its  relation  to 
solar  radiation  have  already  been  discussed  in  connection 
with  volcanoes.  It  will  be  recalled  that  on  the  average  it 
lowers  the  surface  temperature.  At  lower  levels,  since 
dust  absorbs  heat  quickly  and  gives  it  out  quickly,  its 
presence  raises  the  temperature  of  the  air  by  day  and 
lowers  it  by  night.  Hence  an  increase  in  dustiness  tends 
toward  greater  extremes. 

From  all  these  considerations  it  appears  that  if  the 
atmosphere  has  actually  evolved  according  to  the  suppo- 
sition which  is  here  tentatively  entertained,  the  general 
tendency  of  the  resultant  climatic  changes  must  have 
been  partly  toward  long  geological  oscillations  and  partly 
toward  a  general  though  very  slight  increase  in  climatic 
severity  and  in  the  contrasts  between  the  zones.  This 
seems  to  agree  with  the  geological  record,  although  the 
fact  that  we  are  living  in  an  age  of  relative  climatic 
severity  may  lead  us  astray. 

The  significant  fact  about  the  whole  matter  is  that  the 
three  great  types  of  terrestrial  agencies,  namely,  those 
of  the  earth's  interior,  those  of  the  oceans,  and  those  of 
the  air,  all  seem  to  have  suffered  changes  which  lead  to 
slow  variations  of  climate.  Many  reversals  have  doubt- 
less taken  place,  and  the  geologic  oscillations  thus  in- 
duced are  presumably  of  much  greater  importance  than 


OCEANS  AND  ATMOSPHERE  241 

the  progressive  change,  yet  so  far  as  we  can  tell  the 
purely  terrestrial  changes  throughout  the  hundreds  of 
millions  of  years  of  geological  time  have  tended  toward 
complexity  and  toward  increased  contrasts  from  conti- 
nent to  ocean,  from  latitude  to  latitude,  from  season  to 
season,  and  from  day  to  night. 

Throughout  geological  history  the  slow  and  almost 
imperceptible  differentiation  of  the  earth's  surface  has 
been  one  of  the  most  noteworthy  of  all  changes.  It  has 
been  opposed  by  the  extraordinary  conservatism  of  the 
universe  which  causes  the  average  temperature  today  to 
be  so  like  that  of  hundreds  of  millions  of  years  ago  that 
many  types  of  life  are  almost  identical.  Nevertheless,  the 
differentiation  has  gone  on.  Often,  to  be  sure,  it  has  pre- 
sumably been  completely  masked  by  the  disturbances  of 
the  solar  atmosphere  which  appear  to  have  been  the 
cause  of  the  sharper,  shorter  climatic  pulsations.  But 
regardless  of  cosmic  conservatism  and  of  solar  impulses 
toward  change,  the  slow  differentiation  of  the  earth's 
surface  has  apparently  given  to  the  world  of  today  much 
of  the  geographical  complexity  which  is  so  stimulating 
a  factor  in  organic  evolution.  Such  complexity — such 
diversity  from  place  to  place — appears  to  be  largely 
accounted  for  by  purely  terrestrial  causes.  It  may  be 
regarded  as  the  great  terrestrial  contribution  to  the 
climatic  environment  which  guides  the  development  Of 
life. 


CHAPTER  XIV 
THE  EFFECT  OF  OTHER  BODIES  ON  THE  SUN 

IF  solar  activity  is  really  an  important  factor  in 
causing  climatic  changes,  it  behooves  us  to  subject 
the  sun  to  the  same  kind  of  inquiry  to  which  we 
have  subjected  the  earth.  We  have  inquired  into  the  na- 
ture of  the  changes  through  which  the  earth's  crust,  the 
oceans,  and  the  atmosphere  have  influenced  the  climate 
of  geological  times.  It  has  not  been  necessary,  however, 
to  study  the  origin  of  the  earth,  nor  to  trace  its  earlier 
stages.  Our  study  of  the  geological  record  begins  only 
when  the  earth  had  attained  practically  its  present  mass, 
essentially  its  present  shape,  and  a  climate  so  similar  to 
that  of  today  that  life  as  we  know  it  was  possible.  In 
other  words,  the  earth  had  passed  the  stages  of  infancy, 
childhood,  youth,  and  early  maturity,  and  had  reached 
full  maturity.  As  it  still  seems  to  be  indefinitely  far  from 
old  age,  we  infer  that  during  geological  times  its  relative 
changes  have  been  no  greater  than  those  which  a  man 
experiences  between  the  ages  of  perhaps  twenty-five  and 
forty. 

Similar  reasoning  applies  with  equal  or  greater  force 
to  the  sun.  Because  of  its  vast  size  it  presumably  passes 
through  its  stages  of  development  much  more  slowly  than 
the  earth.  In  the  first  chapter  of  this  book  we  saw  that 
the  earth's  relative  uniformity  of  climate  for  hundreds  of 
millions  of  years  seems  to  imply  a  similar  uniformity  in 
solar  activity.  This  accords  with  a  recent  tendency  among 


EFFECT  OF  OTHER  BODIES  ON  THE  SUN    243 

astronomers  who  are  more  and  more  recognizing  that  the 
stars  and  the  solar  system  possess  an  extraordinary  de- 
gree of  conservatism.  Changes  that  once  were  supposed 
to  take  place  in  thousands  of  years  are  now  thought  to 
have  required  millions.  Hence  in  this  chapter  we  shall 
assume  that  throughout  geological  times  the  condition 
of  the  sun  has  been  almost  as  at  present.  It  may  have 
been  somewhat  larger,  or  different  in  other  ways,  but  it 
was  essentially  a  hot,  gaseous  body  such  as  we  see  today 
and  it  gave  out  essentially  the  same  amount  of  energy. 
This  assumption  will  affect  the  general  validity  of  what 
follows  only  if  it  departs  widely  from  the  truth.  With  this 
assumption,  then,  let  us  inquire  into  the  degree  to  which 
the  sun's  atmosphere  has  probably  been  disturbed 
throughout  geological  times. 

In  Earth  and  Sun,  as  already  explained,  a  detailed 
study  has  led  to  the  conclusion  that  cyclonic  storms  are 
influenced  by  the  electrical  action  of  the  sun.  Such  ac- 
tion appears  to  be  most  intense  in  sunspots,  but  appar- 
ently pertains  also  to  other  disturbed  areas  in  the  sun's 
atmosphere.  A  study  of  sunspots  suggests  that  their 
true  periodicity  is  almost  if  not  exactly  identical  with 
that  of  the  orbital  revolution  of  Jupiter,  11.8  years.  Other 
investigations  show  numerous  remarkable  coincidences 
between  sunspots  and  the  orbital  revolution  of  the  other 
planets,  including  especially  Saturn  and  Mercury.  This 
seems  to  indicate  that  there  is  some  truth  in  the  hypothe- 
sis that  sunspots  and  other  related  disturbances  of  the 
solar  atmosphere  owe  their  periodicity  to  the  varying 
effects  of  the  planets  as  they  approach  and  recede  from 
the  sun  in  their  eccentric  orbits  and  as  they  combine  or 
oppose  their  effects  according  to  their  relative  positions. 
This  does  not  mean  that  the  energy  of  the  solar  disturb- 
ances is  supposed  to  come  from  the  planets,  but  merely 


244  CLIMATIC  CHANGES 

that  their  variations  act  like  the  turning  of  a  switch  to 
determine  when  and  how  violently  the  internal  forces  of 
the  sun  shall  throw  the  solar  atmosphere  into  commotion. 
This  hypothesis  is  by  no  means  new,  for  in  one  form  or 
another  it  has  been. ^advocated  by  Wolfer,  Birkeland, 
E.  W.  Brown,  Schuster,  Arctowski,  and  others. 

The  agency  through  which  the  planets  influence  the 
solar  atmosphere  is  not  yet  clear.  The  suggested  agencies 
are  the  direct  pull  of  gravitation,  the  tidal  effect  of  the 
planets,  and  an  electro-magnetic  effect.  In  Earth  and 
Sun  the  conclusion  is  reached  that  the  first  two  are  out 
of  the  question,  a  conclusion  in  which  E.  W.  Brown 
acquiesces.  Unless  some  unknown  cause  is  appealed  to, 
this  leaves  an  electro-magnetic  hypothesis  as  the  only  one 
which  has  a  reasonable  foundation.  Schuster  inclines  to 
this  view.  The  conclusions  set  forth  in  Earth  and  Sun  as 
to  the  electrical  nature  of  the  sun's  influence  on  the  earth 
point  somewhat  in  the  same  direction.  Hence  in  this 
chapter  we  shall  inquire  what  would  happen  to  the  sun, 
and  hence  to  the  earth,  on  their  journey  .through  space, 
if  the  solar  atmosphere  is  actually  subject  to  disturbance 
by  the  electrical  or  other  effects  of  other  heavenly  bodies. 
It  need  hardly  be  pointed  out  that  we  are  here  venturing 
into  highly  speculative  ground,  and  that  the  verity  or 
falsity  of  the  conclusions  reached  in  this  chapter  has 
nothing  to  do  with  the  validity  of  the  reasoning  in  pre- 
vious chapters.  Those  chapters  are  based  on  the  assump- 
tion that  terrestrial  causes  of  climatic  changes  are  sup- 
plemented by  solar  disturbances  which  produce  their 
effect  partly  through  variations  in  temperature  but  also 
through  variations  in  the  intensity  and  paths  of  cyclonic 
storms.  The  present  chapter  seeks  to  shed  some  light  on 
the  possible  causes  and  sequence  of  solar  disturbances. 

Let  us  begin  by  scanning  the  available  evidence  as  to 


EFFECT  OF  OTHER  BODIES  ON  THE  SUN    245 

solar  disturbances  previous  to  the  time  when  accurate 
sunspot  records  are  available.  Two  rather  slender  bits  of 
evidence  point  to  cycles  of  solar  activity  lasting  hundreds 
of  years.  One  of  these  has  already  been  discussed  in 
Chapter  VI,  where  the  climatic  stress  of  the  fourteenth 
century  was  described.  At  that  time  sunspots  are  known 
to  have  been  unusually  numerous,  and  there  were  great 
climatic  extremes.  Lakes  overflowed  in  Central  Asia; 
storms,  droughts,  floods,  and  cold  winters  were  unusually 
severe  in  Europe;  the  Caspian  Sea  rose  with  great 
rapidity;  the  trees  of  California  grew  with  a  vigor  un- 
known for  centuries ;  the  most  terrible  of  recorded  fam- 
ines occurred  in  England  and  India;  the  Eskimos  were 
probably  driven  south  by  increasing  snowiness  in  Green- 
land; and  the  Mayas  of  Yucatan  appear  to  have  made 
their  last  weak  attempt  at  a  revival  of  civilization  under 
the  stimulus  of  greater  storminess  and  less  constant 
rainfall. 

The  second  bit  of  evidence  is  found  in  recent  ex- 
haustive studies  of  periodicities  by  Turner1  and  other 
astronomers.  They  have  sought  every  possible  natural 
occurrence  for  which  a  numerical  record  is  available  for 
a  long  period.  The  most  valuable  records  appear  to  be 
those  of  tree  growth,  Nile  floods,  Chinese  earthquakes, 
and  sunspots.  Turner  reaches  the  conclusion  that  all  four 
types  of  phenomena  show  the  same  periodicity,  namely, 
cycles  with  an  average  length  of  about  260  to  280  years. 
He  suggests  that  if  this  is  true,  the  cycles  in  tree  growth 
and  in  floods,  both  of  which  are  climatic,  are  probably  J 

due  to  a  non-terrestrial  cause.  The  fact  that  the  sunspots 

i  H.  H.  Turner:  On  a  Long  Period  in  Chinese  Earthquake  "Records;  Mon. 
Not.  Eoyal  Astron.  Soc.,  Vol.  79,  1919,  pp.  531-539;  Vol.  80,  1920,  pp.  617-    J/v 
619;  Long  Period  Terms  in  the  Growth  of  Trees;  idem,  pp.  793-808. 


CLIMATIC  CHANGES 

show  similar  cycles  suggests  that  the  sun's  variations 
are  the  cause. 

These  two  bits  of  evidence  are  far  too  slight  to  form 
the  foundation  of  any  theory  as  to  changes  in  solar 
activity  in  the  geological  past.  Nevertheless  it  may  be 
helpful  to  set  forth  certain  possibilities  as  a  stimulus  to 
further  research.  For  example,  it  has  been  suggested  that 
meteoric  bodies  may  have  fallen  into  the  sun  and  caused 
it  suddenly  to  flare  up,  as  it  were.  This  is  not  impossible, 
although  it  does  not  appear  to  have  taken  place  since 
men  became  advanced  enough  to  make  careful  observa- 
tions. Moreover,  the  meteorites  which  now  fall  on  the 
earth  are  extremely  small,  the  average  size  being  com- 
puted as  no  larger  than  a  grain  of  wheat.  The  largest 
ever  found  on  the  earth's  surface,  at  Bacubirito  in 
Mexico,  weighs  only  about  fifty  tons,  while  within  the 
rocks  the  evidences  of  meteorites  are  extremely  scanty 
and  insignificant.  If  meteorites  had  fallen  into  the  sun 
often  enough  and  of  sufficient  size  to  cause  glacial  fluctua- 
tions and  historic  pulsations  of  climate,  it  seems  highly 
probable  that  the  earth  would  show  much  more  evidence 
of  having  been  similarly  disturbed.  And  even  if  the  sun 
should  be  bombarded  by  large  meteors  the  result  would 
probably  not  be  sudden  cold  periods,  which  are  the  most 
notable  phenomena  of  the  earth's  climatic  history,  but 
sudden  warm  periods  followed  by  slow  cooling.  Neverthe- 
less, the  disturbance  of  the  sun  by  collision  with  meteoric 
matter  can  by  no  means  be  excluded  as  a  possible  cause 
of  climatic  variations. 

Allied  to  the  preceding  hypothesis  is  Shapley's2  nebu- 
lar hypothesis.  At  frequent  intervals,  averaging  about 

2  Harlow  Shapley :  Note  on  a  Possible  Factor  in  Geologic  Climates ; 
Jour.  Geol.,  Vol.  29,  No.  4,  May,  1921;  Novae  and  Variable  Stars,  Pub. 
Astron.  Soc.  Pac.,  No.  194,  Aug.,  1921. 


EFFECT  OF  OTHER  BODIES  ON  THE  SUN    247 

once  a  year  during  the  last  thirty  years,  astronomers  have 
discovered  what  are  known  as  novas.  These  are  stars 
which  were  previously  faint  or  even  invisible,  but  which 
flash  suddenly  into  brilliancy.  Often  their  light-giving 
power  rises  seven  or  eight  magnitudes — a  thousand-fold. 
In  addition  to  the  spectacular  novae  there  are  numerous 
irregular  variables  whose  brilliancy  changes  in  every 
ratio  from  a  few  per  cent  up  to  several  magnitudes.  Most 
of  them  are  located  in  the  vicinity  of  nebulae,  as  is  also 
the  case  with  novae.  This,  as  well  as  other  facts,  makes  it 
probable  that  all  these  stars  are  ' '  friction  variables, ' '  as 
Shapley  calls  them.  Apparently  as  they  pass  through  the 
nebulae  they  come  in  contact  with  its  highly  diffuse 
matter  and  thereby  become  bright  much  as  the  earth 
would  become  bright  if  its  atmosphere  were  filled  with 
millions  of  almost  infinitesimally  small  meteorites.  A  star 
may  also  lose  brilliancy  if  nebulous  matter  intervenes 
between  it  and  the  observer.  If  our  sun  has  been  sub- 
jected to  any  of  these  changes  some  sort  of  climatic  effect 
must  have  been  produced. 

In  a  personal  communication  Shapley  amplifies  the 
nebular  climatic  hypothesis  as  follows : 

Within  700  light  years  of  the  sun  in  many  directions  (Taurus, 
Cygnus,  Ophiuehus,  Scorpio)  are  great  diffuse  clouds  of  nebu- 
losity, some  bright,  most  of  them  dark.  The  probability  that  stars 
moving  in  the  general  region  of  such  clouds  will  encounter  this 
material  is  very  high,  for  the  clouds  fill  enormous  volumes  of 
space, — e.g.,  probably  more  than  a  hundred  thousand  cubic  light 
years  in  the  Orion  region,  and  are  presumably  composed  of  rare- 
fied gases  or  of  dust  particles.  Probably  throughout  all  our 
part  of  space  such  nebulosity  exists  (it  is  all  around  us,  we  are 
sure),  but  only  in  certain  regions  is  it  dense  enough  to  affect 
conspicuously  the  stars  involved  in  it.  If  a  star  moving  at  high 
velocity  should  collide  with  a  dense  part  of  such  a  nebulous 


248  CLIMATIC  CHANGES 

cloud,  we  should  probably  have  a  typical  nova.  If  the  relative 
velocity  of  nebulous  material  and  star  were  low  or  moderate,  or 
if  the  material  were  rare,  we  should  not  expect  a  conspicuous 
effect  on  the  star's  light. 

In  the  nebulous  region  of  Orion,  which  is  probably  of  un- 
usually high  density,  there  are  about  100  known  stars,  varying 
between  20%  and  80%  of  their  total  light — all  of  them  irregu- 
larly— some  slowly,  some  suddenly.  Apparently  they  are 
"friction  variables."  Some  of  the  variables  suddenly  lose  40% 
of  their  light  as  if  blanketed  by  nebulous  matter.  In  the  Trifid 
Nebula  there  are  variables  like  those  of  Orion,  in  Messier  8  also, 
and  probably  many  of  the  100  or  so  around  the  Rho  Ophiuchi 
region  belong  to  this  kind. 

I  believe  that  our  sun  could  not  have  been  a  typical  nova,  at 
least  not  since  the  Archeozoic,  that  is  for  perhaps  a  billion  years. 
I  believe  we  have  in  geological  climates  final  proof  of  this,  be- 
cause an  increase  in  the  amount  of  solar  radiation  by  1000  times 
as  in  the  typical  nova,  would  certainly  punctuate  emphatically 
the  life  cycle  on  the  earth,  even  if  the  cause  of  the  nova  would 
not  at  the  same  time  eliminate  the  smaller  planets.  But  the  sun 
may  have  been  one  of  these  miniature  novae  or  friction  vari- 
ables ;  and  I  believe  it  very  probable  that  its  wanderings  through 
this  part  of  space  could  not  long  leave  its  mean  temperature 
unaffected  to  the  amount  of  a  few  per  cent. 

One  reason  we  have  not  had  this  proposal  insisted  upon 
before  is  that  the  data  back  of  it  are  mostly  new — the  Orion 
variables  have  been  only  recently  discovered  and  studied,  the 
distribution  and  content  of  the  dark  nebulas  are  hardly  as  yet 
generally  known. 

This  interesting  hypothesis  cannot  be  hastily  dis- 
missed. If  the  sun  should  pass  through  a  nebula  it  seems 
inevitable  that  there  would  be  at  least  slight  climatic 
effects  and  perhaps  catastrophic  effects  through  the 
action  of  the  gaseous  matter  not  only  on  the  sun  but  on 
the  earth's  own  atmosphere.  As  an  explanation  of  the 


EFFECT  OF  OTHER  BODIES  ON  THE  SUN    249 

general  climatic  conditions  of  the  past,  however,  Shapley 
points  out  that  the  hypothesis  has  the  objection  of  being 
vague,  and  that  nebulosity  should  not  be  regarded  as 
more  than  "a  possible  factor."  One  of  the  chief  difficul- 
ties seems  to  be  the  enormously  wide  distribution  of  as 
yet  undiscovered  nebulous  matter  which  must  be  assumed 
if  any  large  share  of  the  earth's  repeated  climatic  , 
changes  is  to  be  ascribed  to  such  matter.  If  such  matter1 
is  actually  abundant  in  space,  it  is  hard  to  see  how  any 
but  the  nearest  stars  would  be  visible.  Another  objection 
is  that  there  is  no  known  nebulosity  near  at  hand  with 
which  to  connect  the  climatic  vicissitudes  of  the  last 
glacial  period.  Moreover,  the  known  nebulae  are  so  much 
less  numerous  than  stars  that  the  chances  that  the  sun 
will  encounter  one  of  them  are  extremely  slight.  This, 
however,  is  not  an  objection,  for  Shapley  points  out  that 
during  geological  times  the  sun  can  never  have  varied 
as  much  as  do  the  novae,  or  even  as  most  of  the  friction 
variables.  Thus  the  hypothesis  stands  as  one  that  is  worth 
investigating,  but  that  cannot  be  finally  rejected  or  ac- 
cepted until  it  is  made  more  definite  and  until  more  in- 
formation is  available. 

Another  suggested  cause  of  solar  variations  is  the  rela- 
tively sudden  contraction  of  the  sun  such  as  that  which 
sometimes  occurs  on  the  earth  when  continents  are  up- 
lifted and  mountains  upheaved.  It  seems  improbable  that 
this  could  have  occurred  in  a  gaseous  body  like  the  sun. 
Lacking,  as  it  does,  any  solid  crust  which  resists  a  change 
of  form,  the  sun  probably  shrinks  steadily.  Hence  any 
climatic  effects  thus  produced  must  be  extremely  gradual 
and  must  tend  steadily  in  one  direction  for  millions  of 
years. 

Still  another  suggestion  is  that  the  tidal  action  of  the 
stars  and  other  bodies  which  may  chance  to  approach 


250  CLIMATIC  CHANGES 

the  sun's  path  may  cause  disturbances  of  the  solar 
atmosphere.  The  vast  kaleidoscope  of  space  is  never 
quiet.  The  sun,  the  stars,  and  all  the  other  heavenly 
bodies  are  moving,  often  with  enormous  speed.  Hence  the 
effect  of  gravitation  upon  the  sun  must  vary  constantly 
and  irregularly,  as  befits  the  geological  requirements.  In 
the  case  of  the  planets,  however,  the  tidal  effect  does  not 
seem  competent  to  produce  the  movements  of  the  solar 
atmosphere  which  appear  to  be  concerned  in  the  incep- 
tion of  sunspots.  Moreover,  there  is  only  the  most  remote 
probability  that  a  star  and  the  sun  will  approach  near 
enough  to  one  another  to  produce  a  pronounced  gravita- 
tional disturbance  in  the  solar  atmosphere.  For  instance, 
if  it  be  assumed  that  changes  in  Jupiter's  tidal  effect  on 
the  sun  are  the  main  factor  in  regulating  the  present  dif- 
ference between  sunspot  maxima  and  sunspot  minima, 
the  chances  that  a  star  or  some  non-luminous  body  of 
similar  mass  will  approach  near  enough  to  stimulate 
solar  activity  and  thereby  bring  on  glaciation  are  only 
one  in  twelve  billion  years,  as  will  be  explained  below. 
This  seems  to  make  a  gravitational  hypothesis  im- 
possible. 

Another  possible  cause  of  solar  disturbances  is  that 
the  stars  in  their  flight  through  space  may  exert  an 
electrical  influence  which  upsets  the  equilibrium  of  the 
solar  atmosphere.  At  first  thought  this  seems  even  more 
impossible  than  a  gravitational  effect.  Electrostatic 
effects,  however,  differ  greatly  from  those  of  tides.  They 
vary  as  the  diameter  of  a  body  instead  of  as  its  mass; 
their  differentials  also  vary  inversely  as  the  square  of 
the  distance  instead  of  as  the  cube.  Electrostatic  effects 
also  increase  as  the  fourth  power  of  the  temperature  or 
at  least  would  do  so  if  they  followed  the  law  of  black 
bodies ;  they  are  stimulated  by  the  approach  of  one  body 


EFFECT  OF  OTHER  BODIES  ON  THE  SUN    251 

to  another;  and  they  are  cumulative,  for  if  ions  arrive 
from  space  they  must  accumulate  until  the  body  to  which 
they  have  come  begins  to  discharge  them.  Hence,  on  the 
basis  of  assumptions  such  as  those  used  in  the  preceding 
paragraph,  the  chances  of  an  electrical  disturbance  of 
the  solar  atmosphere  sufficient  to  cause  glaciation  on  the 
earth  may  be  as  high  as  one  in  twenty  or  thirty  million 
years.  This  seems  to  put  an  electrical  hypothesis  within 
the  bounds  of  possibility.  Further  than  that  we  cannot 
now  go.  There  may  be  other  hypotheses  which  fit  the  facts 
much  better,  but  none  seems  yet  to  have  been  suggested. 

In  the  rest  of  this  chapter  the  tidal  and  electrical 
hypotheses  of  stellar  action  on  the  sun  will  be  taken  up 
in  detail.  The  tidal  hypothesis  is  considered  because  in 
discussions  of  the  effect  of  the  planets  it  has  hitherto 
held  almost  the  entire  field.  The  electrical  hypothesis  will 
be  considered  because  it  appears  to  be  the  best  yet  sug- 
gested, although  it  still  seems  doubtful  whether  electrical 
effects  can  be  of  appreciable  importance  over  such  vast 
distances  as  are  inevitably  involved.  The  discussion  of 
both  hypotheses  will  necessarily  be  somewhat  technical, 
and  will  appeal  to  the  astronomer  more  than  to  the  lay- 
man. It  does  not  form  a  necessary  part  of  this  book,  for 
it  has  no  bearing  on  our  main  thesis  of  the  effect  of  the 
sun  on  the  earth.  It  is  given  here  because  ultimately  the 
question  of  changes  in  solar  activity  during  geological 
times  must  be  faced. 

In  the  astronomical  portion  of  the  following  discus- 
sion we  shall  follow  Jeans3  in  his  admirable  attempt  at  a 
mathematical  analysis  of  the  motions  of  the  universe. 
Jeans  divides  the  heavenly  bodies  into  five  main  types: 
(1)  Spiral  nebulae,  which  are  thought  by  some  astrono- 

s  J.  H.  Jeans :  Problems  of  Cosmogony  and  Stellar  Dynamics,  Cam- 
bridge, 1919. 


252  CLIMATIC  CHANGES 

j,  mers  to  be  systems  like  our  own  in  the  making,  and  by 
Bothers  to  be  independent  universes  lying  at  vast  distances 
beyond  the  limits  of  our  Galactic  universe,  as  it  is  called 
from  the  Galaxy  or  Milky  Way.  (2)  Nebulae  of  a  smaller 
type,  called  planetary.  These  lie  within  the  Galactic  por- 
tion of  the  universe  and  seem  to  be  early  stages  of  what 
may  some  day  be  stars  or  solar  systems.  (3)  Binary  or 
multiple  stars,  which  are  extraordinarily  numerous.  In 
some  parts  of  the  heavens  they  form  50  or  even  60  per 
cent  of  the  stars  and  in  the  galaxy  as  a  whole  they  seem  to 
form  "fully  one  third."  (4)  Star  clusters.  These  consist 
of  about  a  hundred  groups  of  stars  in  each  of  which  the 
stars  move  together  in  the  same  direction  with  approxi- 
mately the  same  velocity.  These,  like  the  spiral  nebulae, 
are  thought  by  some  astronomers  to  lie  outside  the  limits 
of  the  galaxy,  but  this  is  far  from  certain.  (5)  The  solar 
system.  According  to  Jeans  this  seems  to  be  unique.  It 
does  not  fit  into  the  general  mathematical  theory  by 
which  he  explains  spiral  nebulae,  planetary  nebulae,  binary 
stars,  and  star  clusters.  It  seems  to  demand  a  special 
explanation,  such  as  is  furnished  by  tidal  disruption  due 
to  the  passage  of  the  sun  close  to  another  star. 

The  part  of  Jeans '  work  which  specially  concerns  us  is 
his  study  of  the  probability  that  some  other  star  will 
approach  the  sun  closely  enough  to  have  an  appreciable 
gravitative  or  electrical  effect,  and  thus  cause  disturb- 
ances in  the  solar  atmosphere.  Of  course  both  the  star 
and  the  sun  are  moving,  but  to  avoid  circumlocution  we 
shall  speak  of  such  mutual  approaches  simply  as  ap- 
proaches of  the  sun.  For  our  present  purpose  the  most 
fundamental  fact  may  be  summed  up  in  a  quotation  from 
Jeans  in  which  he  says  that  most  stars  "show  evidence 
of  having  experienced  considerable  disturbance  by  other 
systems ;  there  is  no  reason  why  our  solar  system  should 


EFFECT  OF  OTHER  BODIES  ON  THE  SUN    253 

be  expected  to  have  escaped  the  common  fate."  Jeans 
gives  a  careful  calculation  from  which  it  is  possible  to 
derive  some  idea  of  the  probability  of  any  given  degree  of 
approach  of  the  sun  and  some  other  star.  Of  course  all 
such  calculations  must  be  based  on  certain  assumptions. 
The  assumptions  made  by  Jeans  are  such  as  to  make  the 
probability  of  close  approaches  as  great  as  possible.  For 
example,  he  allows  only  560  million  years  for  the  entire  . 
evolution  of  the  sun,  whereas  some  astronomers  and 
geologists  would  put  the  figure  ten  or  more  times  as 
high.  Nevertheless,  Jeans'  assumptions  at  least  show 
the  order^^magnitiide  which  we  may  expect  on  the  basis 
of  reasonable  astronomical  conclusions. 

According  to  the  planetary  hypothesis  of  sunspots,  the 
difference  in  the  effect  of  Jupiter  when  it  is  nearest  and  - 
farthest  from  the  sun  is  the  main  factor  in  starting  the 
sunspot  cycle  and  hence  the  corresponding  terrestrial' 
cycle.  The  climatic  difference  between  sunspot  maxima 
and  minima,  as  measured  by  temperature,  apparently 
amounts  to  at  least  a  twentieth  and  perhaps  a  tenth  of 
the  difference  between  the  climate  of  the  last  glacial 
epoch  and  the  present.  We  may  suppose,  then,  that  a  body 
which  introduced  a  gravitative  or  electrical  factor  twenty 
times  as  great  as  the  difference  in  Jupiter's  effect  at  its 
maximum  and  minimum  distances  from  the  sun  would 
cause  a  glacial  epoch  if  the  effect  lasted  long  enough.  Of 
course  the  other  planets  combine  their  effects  with  that 
of  Jupiter,  but  for  the  sake  of  simplicity  we  will  leave 
the  others  out  of  account.  The  difference  between  Jupi- 
ter's maximum  and  minimum  tidal  effect  on  the  sun,.  \^ 
amounts  to  29  per  cent  of  the  planet's  average  effect.] \ 
The  corresponding  difference,  according  to  the  electrical 
hypothesis,  is  about  19  per  cent,  for  electrostatic  action 
varies  as  the  square  of  the  distance  instead  of  as  the  cube. 


254  CLIMATIC  CHANGES 

Let  us  assume  that  a  body  exerting  four  times  Jupiter's 
present  tidal  effect  and  placed  at  the  average  distance  of 
Jupiter  from  the  sun  would  disturb  the  sun's  atmosphere 
twenty  times  as  much  as  the  present  difference  between 
sunspot  maxima  and  minima,  and  thus,  perhaps,  cause  a 
glacial  period  on  the  earth. 

On  the  basis  of  this  assumption  our  first  problem  is  to 
estimate  the  frequency  with  which  a  star,  visible  or 
dark,  is  likely  to  approach  near  enough  to  the  sun  to 
produce  a  tidal  effect  four  times  that  of  Jupiter.  The 
number  of  visible  stars  is  known  or  at  least  well  esti- 
mated. As  to  dark  stars,  which  have  grown  cool,  Arrhe- 
nius  believed  that  they  are  a  hundred  times  as  numerous 
as  bright  stars;  few  astronomers  believe  that  there  are 
less  than  three  or  four  times  as  many.  Dr.  Shapley  of 
the  Harvard  Observatory  states  that  a  new  investigation 
of  the  matter  suggests  that  eight  or  ten  is  probably  a 
maximum  figure.  Let  us  assume  that  nine  is  correct. 
The  average  visible  star,  so  far  as  measured,  has  a  mass 
about  twice  that  of  the  sun,  or  about  2100  times  that  of 
Jupiter.  The  distances  of  the  stars  have  been  measured 
in  hundreds  of  cases  and  thus  we  can  estimate  how  many 
stars,  both  visible  and  invisible,  are  on  an  average  con- 
tained in  a  given  volume  of  space.  On  this  basis  Jeans 
estimates  that  there  is  only  one  chance  in  thirty  billion 
years  that  a  visible  star  will  approach  within  2.8  times 
the  distance  of  Neptune  from  the  sun,  that  is,  within  about 
eight  billion  miles.  If  we  include  the  invisible  stars  the 
chances  become  one  in  three  billion  years.  In  order  to 
produce  four  times  the  tidal  effect  of  Jupiter,  however, 
the  average  star  would  have  to  approach  within  about 
four  billion  miles  of  the  sun,  and  the  chances  of  that 
are  only  one  in  twelve  billion  years.  The  disturbing  star 


EFFECT  OF  OTHER  BODIES  ON  THE  SUN    255 

would  be  only  40  per  cent  farther  from  the  sun  than 
Neptune,  and  would  almost  pass  within  the  solar  system. 

Even  though  Jeans  holds  that  the  frequency  of  the 
mutual  approach  of  the  sun  and  a  star  was  probably 
much  greater  in  the  distant  past  than  at  present,  the 
figures  just  given  lend  little  support  to  the  tidal  hypothe- 
sis. In  fact,  they  apparently  throw  it  out  of  court.  It  will 
be  remembered  that  Jeans  has  made  assumptions  which 
give  as  high  a  frequency  of  stellar  encounters  as  is  con- 
sistent with  the  astronomical  facts.  We  have  assumed 
nine  dark  stars  for  every  bright  one,  which  may  be  a 
liberal  estimate.  Also,  although  we  have  assumed  that  a 
disturbance  of  the  sun's  atmosphere  sufficient  to  cause 
a  glacial  period  would  arise  from  a  tidal  effect  only 
twenty  times  as  great  as  the  difference  in  Jupiter's  effect 
when  nearest  the  sun  and  farthest  away,  in  our  computa- 
tions this  has  actually  been  reduced  to  thirteen.  With  all 
these  favorable  assumptions  the  chances  of  a  stellar  ap- 
proach of  the  sort  here  described  are  now  only  one  in 
twelve  billion  years.  Yet  within  a  hundred  million  years, 
according  to  many  estimates  of  geological  time,  and 
almost  certainly  within  a  billion,  there  have  been  at  least 
half  a  dozen  glaciations. 

Our  use  of  Jeans'  data  interposes  another  and  equally 
insuperable  difficulty  to  any  tidal  hypothesis.  Four  bil- 
v/  lion  miles  is  a  very  short  distance  in  the  eyes  of  an 
astronomer.  At  that  distance  a  star  twice  the  size  of  the 
sun  would  attract  the  outer  planets  more  strongly  than 
the  sun  itself,  and  might  capture  them.  If  a  star  should 
come  within  four  billion  miles  of  the  sun,  its  effect  in 
distorting  the  orbits  of  all  the  planets  would  be  great. 
If  this  had  happened  often  enough  to  cause  all  the  gla- 
ciations known  to  geologists,  the  planetary  orbits  would 
be  strongly  elliptical  instead  of  almost  circular.  The  con- 


256  CLIMATIC  CHANGES 

siderations  here  advanced  militate  so  strongly  against 
the  tidal  hypothesis  of  solar  disturbances  that  it  seems 
scarcely  worth  while  to  consider  it  further. 

Let  us  turn  now  to  the  electrical  hypothesis.  Here  the 
conditions  are  fundamentally  different  from  those  of  the 
tidal  hypothesis.  In  the  first  place  the  electrostatic  effect 
of  a  body  has  nothing  to  do  with  its  mass,  but  depends  on 
the  area  of  its  surface ;  that  is,  it  varies  as  the  square  of 
the  radius.  Second,  the  emission  of  electrons  varies  ex- 
ponentially. If  hot  glowing  stars  follow  the  same  law  as 
black  bodies  at  lower  temperatures,  the  emission  of 
electrons,  like  the  emission  of  other  kinds  of  energy, 
varies  as  the  fourth  power  of  the  absolute  temperature. 
In  other  words,  suppose  there  are  two  black  bodies,  other- 
wise alike,  but  one  with  a  temperature  of  27°  C.  or  300° 
on  the  absolute  scale,  and  the  other  with  600°  on  the 
absolute  scale.  The  temperature  of  one  is  twice  as  high 
as  that  of  the  other,  but  the  electrostatic  effect  will  be 
sixteen  times  as  great.4  Third,  the  number  of  electrons 

4  This  fact  is  so  important  and  at  the  same  time  so  surprising  to  the 
layman,  that  a  quotation  from  The  Electron  Theory  of  Matter  by  O.  W. 
Eichardson,  1914,  pp.  326  and  334  is  here  added. 

"It  is  a  very  familiar  fact  that  when  material  bodies  are  heated  they 
emit  electromagnetic  radiations,  in  the  form  of  thermal,  luminous,  and 
actinic  rays,  in  appreciable  quantities.  Such  an  effect  is  a  natural  consequence 
of  the  electron  and  kinetic  theories  of  matter.  On  the  kinetic  theory,  tem- 
perature is  a  measure  of  the  violence  of  the  motion  of  the  ultimate  par- 
ticles; and  we  have  seen  that  on  the  electron  theory,  electromagnetic 
radiation  is  a  consequence  of  their  acceleration.  The  calculation  of  this 
emission  from  the  standpoint  of  the  electron  theory  alone  is  a  very  complex 
problem  which  takes  us  deeply  into  the  structure  of  matter  and  which  has 
probably  not  yet  been  satisfactorily  resolved.  Fortunately,  we  can  find  out 
a  great  deal  about  these  phenomena  by  the  application  of  general  prin- 
ciples like  the  conservation  of  energy  and  the  second  law  of  thermo- 
dynamics without  considering  special  assumptions  about  the  ultimate  con- 
stitution of  matter.  It  is  to  be  borne  in  mind  that  the  emission  under 
consideration  occurs  at  all  temperatures  although  it  is  more  marked  the 
higher  the  temperature.  .  .  .  The  energy  per  unit  volume,  in  vacuo,  of  the 
radiation  in  equilibrium  in  an  enclosure  at  the  absolute  temperature,  T,  is 
equal  to  a  universal  constant,  A,  multiplied  by  the  fourth  power  of  the 


EFFECT  OF  OTHER  BODIES  ON  THE  SUN    257 

that  reach  a  given  body  varies  inversely  as  the  square  of 
the  distance,  instead  of  as  the  cube  which  is  the  case 
with  tide-making  forces. 

In  order  to  use  these  three  principles  in  calculating 
the  effect  of  the  stars  we  must  know  the  diameters,  dis- 
tances, temperature,  and  number  of  the  stars.  The  dis- 
tances and  number  may  safely  be  taken  as  given  by  Jeans 
in  the  calculations  already  cited.  As  to  the  diameters,  the 
measurements  of  the  stars  thus  far  made  indicate  that 
the  average  mass  is  about  twice  that  of  the  sun.  The 
average  density,  as  deduced  by  Shapley5  from  the  move- 
ments of  double  stars,  is  about  one-eighth  the  solar 
density.  This  would  give  an  average  diameter  about  two 
and  a  half  times  that  of  the  sun.  For  the  dark  stars,  we 
shall  assume  for  convenience  that  they  are  ten  times  as 
numerous  as  the  bright  ones.  We  shall  also  assume  that 
their  diameter  is  half  that  of  the  sun,  for  being  cool  they 
must  be  relatively  dense,  and  that  their  temperature  is 
the  same  as  that  which  we  shall  assume  for  Jupiter. 

As  to  Jupiter  we  shall  continue  our  former  assumption 
that  a  body  with  four  times  the  effectiveness  of  that 
planet,  which  here  means  with  twice  as  great  a  radius, 
would  disturb  the  sun  enough  to  cause  glaciation.  It 
would  produce  about  twenty  times  the  electrostatic  effect 

absolute  temperature.  Since  the  intensity  of  the  radiation  is  equal  to  the 
energy  per  unit  volume  multiplied  by  the  velocity  of  light,  it  follows  that 
the  former  must  also  be  proportional  to  the  fourth  power  of  the  absolute 
temperature.  Moreover,  if  E  is  the  total  emission  from  unit  area  of  a 
perfectly  black  body,  we  see  from  p.  330  that  ErrA'T*,  where  A'  is  a  new 
universal  constant.  This  result  is  usually  known  as  Stefan's  Law.  It  was 
suggested  by  Stefan  in  the  inaccurate  form  that  the  total  radiant  energy 
of  emission  from  bodies  varies  as  the  fourth  power  of  the  absolute  tempera- 
ture, as  a  generalization  from  the  results  of  experiments.  The  credit  for 
showing  that  it  is  a  consequence  of  the  existence  of  radiation  pressure 
combined  with  the  principles  of  thermodynamics  is  due  to  Bartoli  and 
Boltzmann. ' ' 

s  Quoted  by  Moulton  in  his  Introduction  to  Astronomy. 


258  CLIMATIC  CHANGES 

which  now  appears  to  be  associated  with  the  difference  in 
Jupiter's  effect  at  maximum  and  minimum.  The  tempera- 
ture of  Jupiter  must  also  be  taken  into  account.  The 
planet  is  supposed  to  be  hot  because  its  density  is  low, 
being  only  about  1.25  that  of  water.  Nevertheless,  it  is 
probably  not  luminous,  for  as  Moulton6  puts  it,  shadows 
upon  it  are  black  and  its  moons  show  no  sign  of  illumina- 
tion except  from  the  sun.  Hence  a  temperature  of  about 
600° C.,  or  approximately  900°  on  the  absolute  scale, 
seems  to  be  the  highest  that  can  reasonably  be  assigned 
to  the  cold  outer  layer  whence  electrons  are  emitted.  As 
to  the  temperature  of  the  sun,  we  shall  adopt  the  common 
estimate  of  about  6300° C.  on  the  absolute  scale.  The 
other  stars  will  be  taken  as  averaging  the  same,  although 
of  course  they  vary  greatly. 

When  Jeans'  method  of  calculating  the  probability  of 
a  mutual  approach  of  the  sun  and  a  star  is  applied  to  the 
assumptions  given  above,  the  results  are  as  shown  in 
Table  5.  On  that  basis  the  dark  stars  seem  to  be  of 
negligible  importance  so  far  as  the  electrical  hypothesis 
is  concerned.  Even  though  they  may  be  ten  times  as 
numerous  as  the  bright  ones  there  appears  to  be  only 
one  chance  in  130  billion  years  that  one  of  them  will  ap- 
proach the  sun  closely  enough  to  cause  the  assumed  dis- 
turbance of  the  solar  atmosphere.  On  the  other  hand,  if 
all  the  visible  stars  were  the  size  of  the  sun,  and  as  hot 
as  that  body,  their  electrical  effect  would  be  fourfold 
that  of  our  assumed  dark  star  because  of  their  size,  and 
2401  times  as  great  because  of  their  temperature,  or  ap- 
proximately 10,000  times  as  great.  Under  such  conditions 
the  theoretical  chance  of  an  approach  that  would  cause 
glaciation  is  one  in  130  million  years.  If  the  average 
visible  star  is  somewhat  cooler  than  the  sun  and  has  a 

«  Introduction  to  Astronomy. 


EFFECT  OF  OTHER  BODIES  ON  THE  SUN    259 

radius  about  two  and  one-half  times  as  great,  as  appears 
to  be  the  fact,  the  chances  rise  to  one  in  thirty-eight  mil- 
lion years.  A  slight  and  wholly  reasonable  change  in  our 
assumptions  would  reduce  this  last  figure  to  only  five  or 
ten  million.  For  instance,  the  earth's  mean  temperature 
during  the  glacial  period  has  been  assumed  as  10°  C. 
lower  than  now,  but  the  difference  may  have  been  only  6°. 
Again,  the  temperature  of  the  outer  atmosphere  of  Jupi- 
ter where  the  electrons  are  shot  out  may  be  only  500°  or 
700°  absolute,  instead  of  900°.  Or  the  diameter  of  the 
average  star  may  be  five  or  ten  times  that  of  the  sun, 
instead  of  only  two  and  one-half  times  as  great.  All  this, 
however,  may  for  the  present  be  disregarded.  The  essen- 
tial point  is  that  even  when  the  assumptions  err  on  the 
side  of  conservatism,  the  results  are  of  an  order  of  magni- 
tude which  puts  the  electrical  hypothesis  within  the 
bounds  of  possibility,  whereas  similar  assumptions  put 
the  tidal  hypothesis,  with  its  single  approach  in  twelve 
billion  years,  far  beyond  those  limits. 

The  figures  for  Betelgeuse  in  Table  5  are  interesting. 
At  a  meeting  of  the  American  Association  for  the  Ad- 
vancement of  Science  in  December,  1920,  Michelson 
reported  that  by  measurements  of  the  interference  of 
light  coming  from  the  two  sides  of  that  bright  star  in 
Orion,  the  observers  at  Mount  Wilson  had  confirmed  the 
recent  estimates  of  three  other  authorities  that  the  star 's 
diameter  is  about  218  million  miles,  or  250  times  that  of 
the  sun.  If  other  stars  so  much  surpass  the  estimates  of 
only  a  decade  or  two  ago,  the  average  diameter  of  all  the 
visible  stars  must  be  many  times  that  of  the  sun.  The  low 
figure  for  Betelgeuse  in  section  D  of  the  table  means  that 
if  all  the  stars  were  as  large  as  Betelgeuse,  several  might 
often  be  near  enough  to  cause  profound  disturbances  of 
the  solar  atmosphere.  Nevertheless,  because  of  the  low 


260 


CLIMATIC  CHANGES 


TABLE  5 

THEORETICAL  PROBABILITY  OF  STELLAR 

APPROACHES 

1 

2 

3 

4 

Average 

Dark  Stars 

Sun 

Star 

Betdgeme 

A.  Approximate 

radius  in  miles 

430,000 

860,000 

2,150,000 

218,000,000 

B.  Assumed  tem- 

perature above 

absolute  zero  .  . 

900°  C. 

6300°  C. 

5400°  C. 

3150°  C. 

C.  Approximate 

theoretical  dis- 

tance at  which 

star  would 

cause  solar  dis- 

turbance great 

enough  to  cause 

glaciation  (bil- 

lions7 of  miles). 

1.2 

120 

220 

3200 

D.  Average  in- 

terval between 

approaches 

close  enough  to 

cause    glacia- 

tion if  all  stars 

were    of   given 

type.     Years.. 

130,000,000,000» 

130,000,000 

38,000,000 

700,000 

temperature  of  the  giant  red  stars  of  the  Betelgeuse  type, 
the  distance  at  which  one  of  them  would  produce  a  given 
electrical  effect  is  only  about  five  times  the  distance  at 
which  our  assumed  average  star  would  produce  the  same 
effect.  This,  to  be  sure,  is  on  the  assumption  that  the 

7  The  term  billions,  here  and  elsewhere,  is  used  in  the  American  sense,  109. 

8  The  assumed  number  of  stars  here  is  ten  times  as  great  as  in  the  other 
parts  of  this  line. 


EFFECT  OF  OTHER  BODIES  ON  THE  SUN    261 

radiation  of  energy  from  incandescent  bodies  varies 
according  to  temperature  in  the  same  ratio  as  the  radia- 
tion from  black  bodies.  Even  if  this  assumption  departs 
somewhat  from  the  truth,  it  still  seems  almost  certain 
that  the  lower  temperature  of  the  red  compared  with  the 
high  temperature  of  the  white  stars  must  to  a  consider- 
able degree  reduce  the  difference  in  electrical  effect  which 
would  otherwise  arise  from  their  size. 

Thus  far  in  our  attempt  to  estimate  the  distance  at 
which  a  star  might  disturb  the  sun  enough  to  cause  gla- 
ciation  on  the  earth,  we  have  considered  only  the  star's 
size  and  temperature.  No  account  has  been  taken  of  the 
degree  to  which  its  atmosphere  is  disturbed.  Yet  in  the 
case  of  the  sun  this  seems  to  be  one  of  the  most  important 
factors.  The  magnetic  field  of  sunspots  is  sometimes  50 
or  100  times  as  strong  as  that  of  the  sun  in  general.  The 
strength  of  the  magnetic  field  appears  to  depend  on  the 
strength  of  the  electrical  currents  in  the  solar  atmos- 
phere. But  the  intensity  of  the  sunspots  and,  by  inference, 
of  the  electrical  currents,  may  depend  on  the  electrical 
action  of  Jupiter  and  the  other  planets.  If  we  apply  a 
similar  line  of  reasoning  to  the  stars,  we  are  at  once  led 
to  question  whether  the  electrical  activity  of  double  stars 
may  not  be  enormously  greater  than  that  of  isolated 
stars  like  the  sun. 

If  this  line  of  reasoning  is  correct,  the  atmosphere  of 
every  double  star  must  be  in  a  state  of  commotion  vastly 
greater  than  that  of  the  sun's  atmosphere  even  when  it 
is  most  disturbed.  For  example,  suppose  the  sun  were 
accompanied  by  a  companion  of  equal  size  at  a  distance 
of  one  million  miles,  which  would  make  it  much  like  many    3W^ 
known  double  stars.  Suppose  also  that  in  accordance  with/" 
the  general  laws  of  physics  the  electrical  effect  of  the 
two  suns  upon  one  another  is  proportional  to  the  fourth 


262  CLIMATIC  CHANGES 

power  of  the  temperature,  the  square  of  the  radius,  and 
the  inverse  square  of  the  distance.  Then  the  effect  of  each 
sun  upon  the  other  would  be  sixty  billion  (6  x  1010)  times 
as  great  as  the  present  electrical  effect  of  Jupiter  upon 
the  sun.  Just  what  this  would  mean  as  to  the  net  effect 
of  a  pair  of  such  suns  upon  the  electrical  potential  of 
other  bodies  at  a  distance  we  can  only  conjecture.  The 
outstanding  fact  is  that  the  electrical  conditions  of  a 
double  star  must  be  radically  different  and  vastly  more 
intense  than  those  of  a  single  star  like  the  sun. 

This  conclusion  carries  weighty  consequences.  At  pres- 
ent twenty  or  more  stars  are  known  to  be  located  within 
about  100  trillion  miles  of  the  sun  (five  par  sees,  as  the 
astronomers  say),  or  16.5  light  years.  According  to  the 
assumptions  employed  in  Table  5  an  average  single  star 
would  influence  the  sun  enough  to  cause  glaciation  if  it 
came  within  approximately  200  billion  miles.  If  the  star 
were  double,  however,  it  might  have  an  electrical  capacity 
enormously  greater  than  that  of  the  sun.  Then  it  would 
be  able  to  cause  glaciation  at  a  correspondingly  great 
distance.  Today  Alpha  Centauri,  the  nearest  known  star, 
is  about  twenty-five  trillion  miles,  or  4.3  light  years  from 
the  sun,  and  Sirius,  the  brightest  star  in  the  heavens,  is 
about  fifty  trillion  miles  away,  or  8.5  light  years.  If  these 
stars  were  single  and  had  a  diameter  three  times  that  of 
the  sun,  and  if  they  were  of  the  same  temperature  as  has 
been  assumed  for  Betelgeuse,  which  is  about  fifty  times  as 
far  away  as  Alpha  Centauri,  the  relative  effects  of  the 
three  stars  upon  the  sun  would  be,  approximately,  Betel- 
geuse 700,  Alpha  Centauri  250,  Sirius  1.  But  Alpha  Cen- 
tauri is  triple  and  Sirius  double,  and  both  are  much  hotter 
than  Betelgeuse.  Hence  Alpha  Centauri  and  even  Sirius 
may  be  far  more  effective  than  Betelgeuse. 

The  two  main  components  of  Alpha  Centauri  are  sepa- 


EFFECT  OF  OTHER  BODIES  ON  THE  SUN    263 

rated  by  an  average  distance  of  about  2,200,000,000  miles, 
or  somewhat  less  than  that  of  Neptune  from  the  sun.  A 
third  and  far  fainter  star,  one  of  the  faintest  yet  meas- 
ured, revolves  around  them  at  a  great  distance.  In  mass 
and  brightness  the  two  main  components  are  about  like 
the  sun,  and  we  will  assume  that  the  same  is  true  of  their 
radius.  Then,  according  to  the  assumptions  made  above, 
their  effect  in  disturbing  one  another  electrically  would 
be  about  10,000  times  the  total  effect  of  Jupiter  upon  the 
sun,  or  2500  times  the  effect  that  we  have  assumed  to  be 
necessary  to  produce  a  glacial  period.  We  have  already 
seen  in  Table  5  that,  according  to  our  assumptions,  a 
single  star  like  the  sun  would  have  to  approach  within 
120  billion  miles  of  the  solar  system,  or  within  2  per  cent 
of  a  light  year,  in  order  to  cause  glaciation.  By  a  similar 
process  of  reasoning  it  appears  that  if  the  mutual  elec- 
trical excitation  of  the  two  main  parts  of  Alpha  Centauri, 
regardless  of  the  third  part,  is  proportional  to  the  ap- 
parent excitation  of  the  sun  by  Jupiter,  Alpha  Centauri 
would  be  5000  times  as  effective  as  the  sun.  In  other 
words,  if  it  came  within  8,500,000,000,000  miles  of  the  sun, 
or  1.4  light  years,  it  would  so  change  the  electrical  condi- 
tions as  to  produce  a  glacial  epoch.  In  that  case  Alpha 
Centauri  is  now  so  near  that  it  introduces  a  disturbing 
effect  equal  to  about  one-sixth  of  the  effect  needed  to 
cause  glaciation  on  the  earth.  Sirius  and  perhaps  others 
of  the  nearer  and  brighter  or  larger  stars  may  also  create 
appreciable  disturbances  in  the  electrical  condition  of  the 
sun's  atmosphere,  and  may  have  done  so  to  a  much 
greater  degree  in  the  past,  or  be  destined  to  do  so  in  the 
future.  Thus  an  electrical  hypothesis  of  solar  disturb- 
ances seems  to  indicate  that  the  position  of  the  sun  in 
respect  to  other  stars  may  be  a  factor  of  great  impor- 
tance in  determining  the  earth 's  climate. 


CHAPTER  XV 
THE  SUN'S  JOURNEY  THROUGH  SPACE 

HAVING  gained  some  idea  of  the  nature  of  the 
electrical  hypothesis  of  solar  disturbances  and 
of  the  possible  effect  of  other  bodies  upon  the 
sun's  atmosphere,  let  us  now  compare  the  astronomical 
data  with  those  of  geology.  Let  us  take  up  five  chief 
points  for  which  the  geologist  demands  an  explanation, 
and  which  any  hypothesis  must  meet  if  it  is  to  be  per- 
manently accepted.  These  are  (1)  the  irregular  intervals 
at  which  glacial  periods  occur;  (2)  the  division  of  glacial 
periods  into  epochs  separated  sometimes  by  hundreds 
of  thousands  of  years;  (3)  the  length  of  glacial  periods 
and  epochs;  (4)  the  occurrence  of  glacial  stages  and  his- 
toric pulsations  in  the  form  of  small  climatic  waves 
superposed  upon  the  larger  waves  of  glacial  epochs;  (5) 
the  occurrence  of  climatic  conditions  much  milder  than 
those  of  today,  not  only  in  the  middle  portion  of  the  great 
geological  eras,  but  even  in  some  of  the  recent  inter- 
glacial  epochs. 

1.  The  irregular  duration  of  the  interval  from  one 
glacial  epoch  to  another  corresponds  with  the  irregular 
distribution  of  the  stars.  If  glaciation  is  indirectly  due 
to  stellar  influences,  the  epochs  might  fall  close  together, 
or  might  be  far  apart.  If  the  average  interval  were  ten 
million  years,  one  interval  might  be  thirty  million  or 
more  and  the  next  only  one  or  two  hundred  thousand. 


THE  SUN'S  JOURNEY  THROUGH  SPACE      265 

According  to  Schuchert,  the  known  periods  of  glacial  or 
semi-glacial  climate  have  been  approximately  as  follows : 


LIST  OF  GLACIAL  PERIODS 

1.  Archeozoic. 

(}4  of  geological  time  or  perhaps  much  more) 
No  known  glacial  periods. 

2.  Proterozoic. 

(*4  of  geological  time) 

a.  Oldest  known  glacial  period  near  base  of  Proterozoic  in 
Canada.  Evidence  widely  distributed. 

b.  Indian  glacial  period;  time  unknown. 

c.  African  glacial  period;  time  unknown. 

d.  Glaciation  near  end  of  Proterozoic  in  Australia,  Norway, 
and  China. 

3.  Paleozoic. 

(V±  of  geological  time) 

a.  Late  Ordovician(?).  Local  in  Arctic  Norway. 

b.  Silurian.  Local  in  Alaska. 

c.  Early  Devonian.  Local  in  South  Africa. 

d.  Early  Permian.  World-wide  and  very  severe. 

4.  Mesozoie  and  Cenozoic. 
(}i  of  geological  time) 

a-b.  None  definitely  determined  during  Mesozoie,  although 
there  appears  to  have  been  periods  of  cooling  (a)  in  the 
late  Triassic,  and  (b)  in  the  late  Cretacic,  with  at  least 
local  glaciation  in  early  Eocene. 

c.  Severe  glacial  period  during  Pleistocene. 


This  table  suggests  an  interesting  inquiry.  During  the 
last  few  decades  there  has  been  great  interest  in  ancient 
glaciation  and  geologists  have  carefully  examined  rocks 
of  all  ages  for  signs  of  glacial  deposits.  In  spite  of  the 
large  parts  of  the  earth  which  are  covered  with  deposits 
belonging  to  the  Mesozoie  and  Cenozoic,  which  form  the 


266  CLIMATIC  CHANGES 

last  quarter  of  geological  time,  the  only  signs  of  actual 
glaciation  are  those  of  the  great  Pleistocene  period  and  a 
few  local  occurrences  at  the  end  of  the  Mesozoic  or  be- 
ginning of  the  Cenozoic.  Late  in  the  Triassic  and  early 
in  the  Jurassic,  the  climate  appears  to  have  been  rigor- 
ous, although  no  tillites  have  been  found  to  demonstrate 
glaciation.  In  the  preceding  quarter,  that  is,  the  Paleo- 
zoic, the  Permian  glaciation  was  more  severe  than  that 
of  the  Pleistocene,  and  the  Devonian  than  that  of  the 
Eocene,  while  the  Ordovician  evidences  of  low  tempera- 
ture are  stronger  than  those  at  the  end  of  the  Triassic. 
In  view  of  the  fact  that  rocks  of  Paleozoic  age  cover 
much  smaller  areas  than  do  those  of  later  age,  the  three 
Paleozoic  glaciations  seem  to  indicate  a  relative  fre- 
quency of  glaciation.  Going  back  to  the  Proterozoic,  it 
is  astonishing  to  find  that  evidence  of  two  highly  de- 
veloped glacial  periods,  and  possibly  four,  has  been  dis- 
covered. Since  the  Indian  and  the  African  glaciations  of 
Proterozoic  times  are  as  yet  undated,  we  cannot  be  sure 
that  they  are  not  of  the  same  date  as  the  others.  Never- 
theless, even  two  is  a  surprising  number,  for  not  only 
are  most  Proterozoic  rocks  so  metamorphosed  that  pos- 
sible evidences  of  glacial  origin  are  destroyed,  but  rocks 
of  that  age  occupy  far  smaller  areas  than  either  those 
of  Paleozoic  or,  still  more,  Mesozoic  and  Cenozoic  age. 
Thus  the  record  of  the  last  three-quarters  of  geological 
time  suggests  that  if  rocks  of  all  ages  were  as  abundant 
and  as  easily  studied  as  those  of  the  later  periods,  the 
frequency  of  glacial  periods  would  be  found  to  increase 
as  one  goes  backward  toward  the  beginnings  of  the 
earth's  history.  This  is  interesting,  for  Jeans  holds  that 
the  chances  that  the  stars  would  approach  one  another 
were  probably  greater  in  the  past  than  at  present.  This 
conclusion  is  based  on  the  assumption  that  our  universe 


THE  SUN'S  JOURNEY  THROUGH  SPACE      267 

is  like  the  spiral  nebulas  in  which  the  orbits  of  the  various 
members  are  nearly  circular  during  the  younger  stages. 
Jeans  considers  it  certain  that  in  such  cases  the  orbits 
will  gradually  become  larger  and  more  elliptical  because 
of  the  attraction  of  one  body  for  another.  Thus  as  time 
goes  on  the  stars  will  be  more  widely  distributed  and 
the  chances  of  approach  will  diminish.  If  this  is  correct, 
the  agreement  between  astronomical  theory  and  geologi- 
cal conclusions  suggests  that  the  two  are  at  least  not  in 
opposition. 

The  first  quarter  of  geological  time  as  well  as  the  last 
three  must  be  considered  in  this  connection.  During  the 
Archeozoic,  no  evidence  of  glaciation  has  yet  been  dis- 
covered. This  suggests  that  the  geological  facts  disprove 
the  astronomical  theory.  But  our  knowledge  of  early 
geological  times  is  extremely  limited,  so  limited  that 
lack  of  evidence  of  glaciation  in  the  Archeozoic  may  have 
no  significance.  Archeozoic  rocks  have  been  studied 
minutely  over  a  very  small  percentage  of  the  earth's  land 
surface.  Moreover,  they  are  highly  metamorphosed  so 
that,  even  if  glacial  tills  existed,  it  would  be  hard  to 
recognize  them.  Third,  according  to  both  the  nebular  and 
the  planetesimal  hypotheses,  it  seems  possible  that 
during  the  earliest  stages  of  geological  history  the 
earth's  interior  was  somewhat  warmer  than  now,  and  the 
surface  may  have  been  warmed  more  than  at  present  by 
conduction,  by  lava  flows,  and  by  the  fall  of  meteorites. 
If  the  earth  during  the  Archeozoic  period  emitted  enough 
heat  to  raise  its  surface  temperature  a  few  degrees,  the 
heat  would  not  prevent  the  development  of  low  forms  of 
life  but  might  effectively  prevent  all  glaciation.  This 
does  not  mean  that  it  would  prevent  changes  of  climate, 
but  merely  changes  so  extreme  that  their  record  would 
be  preserved  by  means  of  ice.  It  will  be  most  interesting 


268  CLIMATIC  CHANGES 

to  see  whether  future  investigations  in  geology  and 
astronomy  indicate  either  a  semi-uniform  distribution  of 
glacial  periods  throughout  the  past,  or  a  more  or  less 
regular  decrease  in  frequency  from  early  times  down  to 
the  present. 

2.  The  Pleistocene  glacial  period  was  divided  into  at 
least  four  epochs,  while  in  the  Permian  at  least  one 
inter-glac;al  epoch  seems  certain,  and  in  some  places  the 
alternation  between  glacial  and  non-glacial  beds  suggests 
no  less  than  nine.  In  the  other  glaciations  the  evidence  is 
not  yet  clear.  The  question  of  periodicity  is  so  important 
that  it  overthrows  most  glacial  hypotheses.  Indeed,  had 
their  authors  known  the  facts  as  established  in  recent 
years,  most  of  the  hypotheses  would  never  have  been 
advanced.  The  carbon  dioxide  hypothesis  is  the  only  one 
which  was  framed  with  geologically  rapid  climatic  alter- 
nations in  mind.  It  certainly  explains  the  facts  of  perio- 
dicity better  than  does  any  of  its  predecessors,  but  even 
so  it  does  not  account  for  the  intimate  way  in  which 
variations  of  all  degrees  from  those  of  the  weather  up  to 
glacial  epochs  seem  to  grade  into  one  another. 

According  to  our  stellar  hypothesis,  occasional  groups 
of  glacial  epochs  would  be  expected  to  occur  close  to- 
gether and  to  form  long  glacial  periods.  This  is  because 
many  of  the  stars  belong  to  groups  or  clusters  in  which 
the  stars  move  in  parallel  paths.  A  good  example  is  the 
cluster  in  the  Hyades,  where  Boss  has  studied  thirty-nine 
stars  with  special  care.1  The  stars  are  grouped  about  a 
center  about  130  light  years  from  the  sun.  The  stars 
themselves  are  scattered  over  an  area  about  thirty 
light  years  in  diameter.  They  average  about  the  same 
distance  apart  as  do  those  near  the  sun,  but  toward  the 

i  Lewis  Boss :  Convergent  of  a  Moving  Cluster  in  Taurus ;  Astronom. 
Jour.,  Vol.  26,  No.  4,  1908,  pp.  31-36. 


THE  SUN'S  JOURNEY  THROUGH  SPACE      269 

center  of  the  group  they  are  somewhat  closer  together. 
The  whole  thirty-nine  sweep  forward  in  essentially 
parallel  paths.  Boss  estimates  that  800,000  years  ago 
the  cluster  was  only  half  as  far  from  the  sun  as  at  pres- 
ent, but  probably  that  was  as  near  as  it  has  been  during 
recent  geological  times.  All  of  the  thirty-nine  stars  of  this 
cluster,  as  Moulton2  puts  it,  "are  much  greater  in  light- 
giving  power  than  the  sun.  The  luminosities  of  even  the 
five  smallest  are  from  five  to  ten  times  that  of  the  sun, 
while  the  largest  are  one  hundred  times  greater  in  light- 
giving  power  than  our  own  luminary.  Their  masses  are 
probably  much  greater  than  that  of  the  sun. ' '  If  the  sun 
were  to  pass  through  such  a  cluster,  first  one  star  and 
then  another  might  come  so  near  as  to  cause  a  profound 
disturbance  in  the  sun's  atmosphere. 

3.  Another  important  point  upon  which  a  glacial  hy- 
pothesis may  come  to  grief  is  the  length  of  the  periods 
or  rather  of  the  epochs  which  compose  the  periods. 
During  the  last  or  Pleistocene  glacial  period  the  evidence 
in  America  and  Europe  indicates  that  the  inter-glacial 
epochs  varied  in  length  and  that  the  later  ones  were 
shorter  than  the  earlier.  Chamberlin  and  Salisbury,  from 
a  comparison  of  various  authorities,  estimate  that  the 
intervals  from  one  glacial  epoch  to  another  form  a  de- 
clining series,  which  may  be  roughly  expressed  as  fol- 
lows: 16-8-4-2-1,  where  unity  is  the  interval  from  the 
climax  of  the  late  Wisconsin,  or  last  glacial  epoch,  to  the 
present.  Most  authorities  estimate  the  culmination  of  the 
late  Wisconsin  glaciation  as  twenty  or  thirty  thousand 
years  ago.  Penck  estimates  the  length  of  the  last  inter- 
glacial  period  as  60,000  years  and  the  preceding  one  as 
240,000.3  R.  T.  Chamberlin,  as  already  stated,  finds  that 

2  F.  E.  Moulton :  in  Introduction  to  Astronomy,  1916. 
a  A.  Penck:  Die  Alpen  im  Eiszeitalter,  Leipzig,  1909. 


270  CLIMATIC  CHANGES 

the  consensus  of  opinion  is  that  inter-glacial  epochs  have 
averaged  five  times  as  long  as  glacial  epochs.  The  actual 
duration  of  the  various  glaciations  probably  did  not  vary 
in  so  great  a  ratio  as  did  the  intervals  from  one  glacia- 
tion  to  another.  The  main  point,  however,  is  the  irregu- 
larity of  the  various  periods. 

The  relation  of  the  stellar  electrical  hypothesis  to  the 
length  of  glacial  epochs  may  be  estimated  from  column 
C,  in  Table  5.  There  we  see  that  the  distances  at  which 
a  star  might  possibly  disturb  the  sun  enough  to  cause 
glaciation  range  all  the  way  from  120  billion  miles  in 
the  case  of  a  small  star  like  the  sun,  to  3200  billion  in 
the  case  of  Betelgeuse,  while  for  double  stars  the  figure 
may  rise  a  hundred  times  higher.  From  this  we  can  cal- 
culate how  long  it  would  take  a  star  to  pass  from  a  point 
where  its  influence  would  first  amount  to  a  quarter  of  the 
assumed  maximum  to  a  similar  point  on  the  other  side  of 
the  sun.  In  making  these  calculations  we  will  assume  that 
the  relative  rate  at  which  the  star  and  the  sun  approach 
each  other  is  about  twenty-two  miles  per  second,  or  700 
million  miles  per  year,  which  is  the  average  rate  of 
motion  of  all  the  known  stars.  According  to  the  distances 
in  Table  5  this  gives  a  range  from  about  500  years  up  to 
about  10,000,  which  might  rise  to  a  million  in  the  case  of 
double  stars.  Of  course  the  time  might  be  relatively  short 
if  the  sun  and  a  rapidly  moving  star  were  approaching 
one  another  almost  directly,  or  extremely  long  if  the  sun 
and  the  star  were  moving  in  almost  the  same  direction 
and  at  somewhat  similar  rates, — a  condition  more 
common  than  the  other.  Here,  as  in  so  many  other  cases, 
the  essential  point  is  that  the  figures  which  we  thus  ob- 
tain seem  to  be  of  the  right  order  of  magnitude. 

4.  Post-glacial  climatic  stages  are  so  well  known  that 
in  Europe  they  have  definite  names.  Their  sequence  has 


THE  SUN'S  JOURNEY  THROUGH  SPACE      271 

already  been  discussed  in  Chapter  XII.  Fossils  found  in 
the  peat  bogs  of  Denmark  and  Scandinavia,  for  example, 
prove  that  since  the  final  disappearance  of  the  conti- 
nental ice  cap  at  the  close  of  the  Wisconsin  there  has 
been  at  least  one  period  when  the  climate  of  Europe  was 
distinctly  milder  than  now.  Directly  overlying  the  sheets 
of  glacial  drift  laid  down  by  the  ice  there  is  a  flora  corre- 
sponding to  that  of  the  present  tundras.  Next  come  re- 
mains of  a  forest  vegetation  dominated  by  birches  and 
poplars,  showing  that  the  climate  was  growing  a  little 
warmer.  Third,  there  follow  evidences  of  a  still  more 
favorable  climate  in  the  form  of  a  forest  dominated  by 
pines ;  fourth,  one  where  oak  predominates ;  and  fifth,  a 
flora  similar  to  that  of  the  Black  Forest  of  Germany, 
indicating  that  in  Scandinavia  the  temperature  was  then 
decidedly  higher  than  today.  This  fifth  flora  has  retreated 
southward  once  more,  having  been  driven  back  to  its 
present  latitude  by  a  slight  recurrence  of  a  cool  stormy 
climate.4  In  central  Asia  evidence  of  post-glacial  stages 
is  found  not  only  in  five  distinct  moraines  but  in  a  corre- 
sponding series  of  elevated  strands  surrounding  salt 
lakes  and  of  river  terraces  in  non-glaciated  arid  regions.5 
In  historic  as  well  as  prehistoric  times,  as  we  have 
already  seen,  there  have  been  climatic  fluctuations.  For 
instance,  the  twelfth  or  thirteenth  century  B.  C.  appears 
to  have  been  almost  as  mild  as  now,  as  does  the  seventh 
century  B.  C.  On  the  other  hand  about  1000  B.  C.,  at  the 
time  of  Christ,  and  in  the  fourteenth  century  there  were 
times  of  relative  severity.  Thus  it  appears  that  both  on 

*  E.  D.  Salisbury :  Physical  Geography  of  the  Pleistocene,  in  Outlines  of 
Geologic  History,  by  Willis  and  Salisbury,  1910,  pp.  273-274. 

s  Davis,  Pumpelly,  and  Huntington :  Explorations  in  Turkestan,  Carnegie 
Inst.  of  Wash.,  No.  26,  1905. 

In  North  America  the  stages  have  been  the  subject  of  intensive  studies 
on  the  part  of  Taylor,  Leverett,  Goldthwait,  and  many  others. 


272  CLIMATIC  CHANGES 

a  large  and  on  a  small  scale  pulsations  of  climate  are  the 
rule.  Any  hypothesis  of  climatic  changes  must  satisfy 
the  periods  of  these  pulsations.  These  conditions  furnish 
a  problem  which  makes  difficulty  for  almost  all  hypothe- 
ses of  climatic  change.  According  to  the  present  hypothe- 
sis, earth  movements  such  as  are  discussed  in  Chapter 
XII  may  cooperate  with  two  astronomical  factors.  One  is 
the  constant  change  in  the  positions  of  the  stars,  a  change 
which  we  have  already  called  kaleidoscopic,  and  the  other 
is  the  fact  that  a  large  proportion  of  the  stars  are  double 
or  multiple.  When  one  star  in  a  group  approaches  the 
sun  closely  enough  to  cause  a  great  solar  disturbance, 
numerous  others  may  approach  or  recede  and  have  a 
minor  effect.  Thus,  whenever  the  sun  is  near  groups  of 
stars  we  should  expect  that  the  earth  would  show  many 
minor  climatic  pulsations  and  stages  which  might  or 
might  not  be  connected  with  glaciation.  The  historic 
pulsations  shown  in  the  curve  of  tree  growth  in  Cali- 
fornia, Fig.  4,  are  the  sort  of  changes  that  would  be 
expected  if  movements  of  the  stars  have  an  effect  on  the 
solar  atmosphere. 

Not  only  are  fully  a  third  of  all  the  visible  stars  double, 
as  we  have  already  seen,  but  at  least  a  tenth  of  these  are 
known  to  be  triple  or  multiple.  In  many  of  the  double 
stars  the  two  bodies  are  close  together  and  revolve  so 
rapidly  that  whatever  periodicity  they  might  create  in 
the  sun's  atmosphere  would  be  very  short.  In  the  triplets, 
however,  the  third  star  is  ordinarily  at  least  ten  times 
as  far  from  the  other  two  as  they  are  from  each  other, 
and  its  period  of  rotation  sometimes  runs  into  hundreds 
or  thousands  of  years.  An  actual  multiple  star  in  the 
constellation  Polaris  will  serve  as  an  example.  The  main 
star  is  believed  by  Jeans  to  consist  of  two  parts  which 
are  almost  in  contact  and  whirl  around  each  other  with 


THE  SUN'S  JOURNEY  THROUGH  SPACE      273 

extraordinary  speed  in  four  days.  If  this  is  true  they 
must  keep  each  other's  atmospheres  in  a  state  of  intense 
commotion.  Much  farther  away  a  third  star  revolves 
around  this  pair  in  twelve  years.  At  a  much  greater  dis- 
tance a  fourth  star  revolves  around  the  common  center 
of  gravity  of  itself  and  the  other  three  in  a  period  which 
may  be  20,000  years.  Still  more  complicated  cases  prob- 
ably exist.  Suppose  such  a  system  were  to  traverse  a 
path  where  it  would  exert  a  perceptible  influence  on  the 
sun  for  thirty  or  forty  thousand  years.  The  varying 
movements  of  its  members  would  produce  an  intricate 
series  of  cycles  which  might  show  all  sorts  of  major  and 
minor  variations  in  length  and  intensity.  Thus  the  varied 
and  irregular  stages  of  glaciation  and  the  pulsations  of 
historic  times  might  be  accounted  for  on  the  hypothesis 
of  the  proximity  of  the  sun  to  a  multiple  star,  as  well  as 
on  that  of  the  less  pronounced  approach  and  recession 
of  a  number  of  stars.  In  addition  to  all  this,  an  almost 
infinitely  complex  series  of  climatic  changes  of  long  and 
short  duration  might  arise  if  the  sun  passed  through  a 
nebula. 

5.  We  have  seen  in  Chapter  VIII  that  the  contrast 
between  the  somewhat  severe  climate  of  the  present  and 
the  generally  mild  climate  of  the  past  is  one  of  the  great 
geological  problems.  The  glacial  period  is  not  a  thing 
of  the  distant  past.  Geologists  generally  recognize  that 
it  is  still  with  us.  Greenland  and  Antarctica  are  both 
shrouded  in  ice  sheets  in  latitudes  where  fossil  floras 
prove  that  at  other  periods  the  climate  was  as  mild  as 
in  England  or  even  New  Zealand.  The  present  glaciated 
regions,  be  it  noted,  are  on  the  polar  borders  of  the 
world's  two  most  stormy  oceanic  areas,  just  where  ice 
would  be  expected  to  last  longest  according  to  the  solar 
cyclonic  hypothesis.  In  contrast  with  the  semi-glacial 


274  CLIMATIC  CHANGES 

conditions  of  the  present,  the  last  inter-glacial  epoch  was 
so  mild  that  not  only  men  but  elephants  and  hippopota- 
muses flourished  in  central  Europe,  while  at  earlier  times 
in  the  middle  of  long  eras,  such  as  the  Paleozoic  and 
Mesozoic,  corals,  cycads,  and  tree  ferns  flourished  within 
the  Arctic  circle. 

If  the  electro-stellar  hypothesis  of  solar  disturbances 
proves  well  founded,  it  may  explain  these  peculiarities. 
Periods  of  mild  climate  would  represent  a  return  of  the 
sun  and  the  earth  to  their  normal  conditions  of  quiet.  At 
such  times  the  atmosphere  of  the  sun  is  assumed  to  be 
little  disturbed  by  sunspots,  faculae,  prominences,  and 
other  allied  evidences  of  movements ;  and  the  rice-grain 
structure  is  perhaps  the  most  prominent  of  the  solar 
markings.  The  earth  at  such  times  is  supposed  to  be 
correspondingly  free  from  cyclonic  storms.  Its  winds  are 
then  largely  of  the  purely  planetary  type,  such  as  trade 
winds  and  westerlies.  Its  rainfall  also  is  largely  planet- 
ary rather  than  cyclonic.  It  falls  in  places  such  as  the 
heat  equator  where  the  air  rises  under  the  influence  of 
heat,  or  on  the  windward  slopes  of  mountains,  or  in  re- 
gions where  warm  winds  blow  from  the  ocean  over  cold 
lands. 

According  to  the  electro-stellar  hypothesis,  the  condi- 
tions which  prevailed  during  hundreds  of  millions  of 
years  of  mild  climate  mean  merely  that  the  solar  system 
was  then  in  parts  of  the  heavens  where  stars — especially 
double  stars — were  rare  or  small,  and  electrical  disturb- 
ances correspondingly  weak.  Today,  on  the  other  hand, 
the  sun  is  fairly  near  a  number  of  stars,  many  of  which 
are  large  doubles.  Hence  it  is  supposed  to  be  disturbed, 
although  not  so  much  as  at  the  height  of  the  last  glacial 
epoch. 

After   the   preceding   parts   of   this   book   had   been 


THE  SUN'S  JOURNEY  THROUGH  SPACE      275 

written,  the  assistance  of  Dr.  Schlesinger  made  it  pos- 
sible to  test  the  electro-stellar  hypothesis  by  comparing 
actual  astronomical  dates  with  the  dates  of  climatic  or 
solar  phenomena.  In  order  to  make  this  possible,  Dr. 
Schlesinger  and  his  assistants  have  prepared  Table  6, 
giving  the  position,  magnitude,  and  motions  of  the  thirty- 
eight  nearest  stars,  and  especially  the  date  at  which  each 
was  nearest  the  sun.  In  column  10  where  the  dates  are 
given,  a  minus  sign  indicates  the  past  and  a  plus  sign  the 
future.  Dr.  Shapley  has  kindly  added  column  12,  giving 
the  absolute  magnitudes  of  the  stars,  that  of  the  sun 
being  4.8,  and  column  13,  showing  their  luminosity  or 
absolute  radiation,  that  of  the  sun  being  unity.  Finally, 
column  14  shows  the  effective  radiation  received  by  the 
sun  from  each  star  when  the  star  is  at  a  minimum  dis- 
tance. Unity  in  this  case  is  the  effect  of  a  star  like  the 
sun  at  a  distance  of  one  light  year. 

It  is  well  known  that  radiation  of  all  kinds,  including 
light,  heat,  and  electrical  emissions,  varies  in  direct  pro- 
portion to  the  exposed  surface,  that  is,  as  the  square 
of  the  radius  of  a  sphere,  and  inversely  as  the  square  of 
the  distance.  From  black  bodies,  as  we  have  seen,  the 
total  radiation  varies  as  the  fourth  power  of  the  abso- 
lute temperature.  It  is  not  certain  that  either  light  or 
electrical  emissions  from  incandescent  bodies  vary  in 
quite  this  same  proportion,  nor  is  it  yet  certain  whether 
luminous  and  electrical  emissions  vary  exactly  together. 
Nevertheless  they  are  closely  related.  Since  the  light 
coming  from  each  star  is  accurately  measured,  while  no 
information  is  available  as  to  electrical  emissions,  we 
have  followed  Dr.  Shapley 's  suggestion  and  used  the 
luminosity  of  the  stars  as  the  best  available  measure  of 
total  radiation.  This  is  presumably  an  approximate 
measure  of  electrical  activity,  provided  some  allowance 


Groombr.  34        

5 
0» 

1 

THIRTY-EIGHT 

(1)                (2)             (3) 

h  h  « 

12m.7      +43°27'          8.1 
43    .0      +57  17           3.6 
43   .9      +4  55         12.3 
12   .4      -69  24           5.0 
39  .4      -16  28           3.6 

TABLE 
STARS  HAVING 

(4)           (5)          (6) 

1     U   if* 

I       IS     111 

Ma         2".  89      +    3 
F8          1  .24      +  10 
FO          3  .01      
F8              .39      +  12 
KO          1  .92      -  16 

i)  Cassiop 

K  Tucante 

T  Ceti 

* 

§2  Eridani  

3 

4 
5 

15 
28 
10 

7 

26 

.9 
.2 
.7 
.7 
.4 

-43 
-  9 

-  7 
-44 
-  3 

27 

48 
49 
59 

42 

4.3 
3.8 
4.5 
9.2 

8.8 

G5 
KO 
G5 
K2 
K2 

3 

4 

8 
2 

.16 

.97 
.08 
.75 
.22 

+  87 
+  16 
-  42 
+242 

e  Eridani  
40(0)2  Eridani  

Cordoba  Z.  243  
Weisse  592  

• 

a  Can.  Maj.  (Sirius).  .. 
a  Can.  Min.  (Procyon). 
Fedorenko  1457-8  
Groombr.  1618  
Weisse  234  

G 
7 
9 

10 

40 
34 
7 
5 

14 

.7 
.1 
.6 
.3 
.2 

-16 

+  5 
+53 
+49 
+20 

35 
29 

7 
58 
22 

-1.6 
0.5 
7.9 
6.8 
9.0 

AO 
F5 
Ma 

K5p 

1 
1 
1 
1 

.32 
.24 
.68 
.45 
.49 

-     8 
-     4 
+  10 
-  30 

1 

Lalande  21185 

11 

13 
14 

57 
0 

12 
40 

32 

.9 
.5 
.0 
.7 

.8 

+36 
+44 
-57 
+15 
-60 

38 
2 
2 
26 
25 

7.6 

8.5 
12.0 

8.5 
0.2 

Mb 
K5 

K5 
G 

4 

4 
2 
2 
3 

.78 
.52 
.69 
.30 
.68 

-  87 
+  65 

+  22 

Lalande  21258  

Lalande  25372  

a  Centauri 

£  Bootes  

14 

16 
17 

46 
51 
41 
11 
12 

.8 
.6 
.4 
.5 
.1 

+19 
-20 
+33 
-46 
-34 

31 

58 
41 
32 
53 

4.6 

5.8 
8.4 
5.7 
5.9 

K5p 
Kp 

K 
K5 

1 
1 

.17 
.96 
.37 
.97 
.19 

+    * 

+  20 

-     4 

Lalande  27173  
Weisse  1259  
Lacaille7194  
/3416.. 

Argel  -0.17415-6  
Barnard's  star  
70p  Ophiuchi   

S  2398 

18 

19 

37 
52 
0 

41 

32 

.0 
.9 
.4 
.7 
..5 

+68 
+  4 
+  2 
+59 
+69 

26 
25 
31 
29 
29 

9.1 
9.7 
4.3 

8.8 
4.8 

K 
Mb 
K 
K 
G5 

1 
10 

1 
2 

1 

.33 
.30 
.13 
.31 

.84 

-'so 

+  26 

a  Draconis    

a  Aquilse  (Altair)   
f61  Cygni   

21 
22 

45 
n 

11 
55 

24 

.9 
.4 
.4 
.7 
.4 

+  8 
+38 
-39 
-57 
+57 

36 
15 
15 
12 
12 

1.2 
5.6 
6.6 

4.8 
9.2 

A5 
K5 
G 
K5 

5 
3 

4 

.66 
.20 
'.53 
.70 

.87 

-  33 

-  64 
+  13 
-  39 

LacailleS760  
e  Indi       

fKriiger  60 

Laeaille  9352  

23 

59 
44 
59 

.4 
.0 
.5 

-36 

+  1 
-37 

26 
52 
51 

7.1 

8.7 
8.2 

K 
Ma 
G 

6 
1 

.90 
.39 
.05 

+  12 

+  "26 

Lalande  46650 

C.  G.  A.  32416  
*  Double  star. 

6 

LARGEST  KNOWN  PARALLAXES 

(7) 

(*) 

(9) 

(10) 

(11) 

(12) 

(13) 

(14) 

Present 
Parallax 

IT 

Maximum 
Parallax 

Minimum 
Distance 
Light  Yrs. 

il! 

Magnitude 
at  Min.  Dist. 

Absolute 
Magnitude 

Luminosity 

IJJJI 

".28 

a  .28 

11.6 

-  4000 

8.1 

10.3 

0.0063 

0.000051 

.18 

.19 

17.1 

-  47000 

3.5 

4.9 

0.91 

0.003110 

.24 

14.2 

0.00017 

.16 

.23 

14.2 

-264000 

4.2 

6.0 

0.33 

0.001610 

.32 

.37 

8.8 

+  46000 

3.3 

6.1 

0.30 

0.003840 

.16 

.22 

14.8 

-  33000 

3.6 

5.3 

0.63 

0.002960 

.31«X 

.46 

7.1 

-106000 

3.0 

6.3 

0.25 

0.004970 

.21 

.23 

14.2 

+  19000 

4.3 

6.1 

0.30 

0.001470 

.32^ 

.68 

4.8 

-  10000 

7.6 

11.7 

0.0017 

0.000074 

.17 

9.9 

0.009 

.37 

.41 

8.0 

4-  65000 

-1.8 

1.2 

27.50 

0.429000  * 

.31 

.32 

10.2 

+  34000 

0.5 

3.0 

5.25 

0.051300  •" 

.16 

.16 

20.4 

-  24000 

7.9 

8.9 

0.023 

0.000055 

.18 

.23 

14.2 

+  69000 

6.3 

8.1 

0.048 

0.000238 

.19 

10.4 

0.0057 

.41  • 

.76 

4.3 

+  20000 

6.2 

10.7 

0.0044 

0.000238 

.19 

.22 

14.8 

-  20000 

8.2 

9.9 

0.009 

0.000041 

34 

14.7 

0.00011 

.19 

9.9 

0.009 

.76*" 

1.03 

3.2 

-  28000 

-0.5 

4.6 

1.20 

0.117500  •• 

.17 

.22 

14.8 

-598000 

4.0 

5.8 

0.40 

0.001815 

.18 

.19 

17.1 

-  36000 

5.6 

7.1 

0.12 

0.000412 

18 

9.7 

0.011 

19 

7.1 

0.12 

.17 

.17 

19.2 

+  21000 

5.7 

7.1 

0.12 

0.000329 

22 

10.8 

0.004 

.53 

.70 

4.7 

+  10000 

9.1 

13.3 

0.0025 

0.000114 

19 

5.7 

0.44 

29 

11.1 

0.0030 

.20 

.23 

14.2 

-  49000 

4.5 

6.3 

0.25 

0.001238 

.21  ^ 

.51 

6.4 

+117000 

-0.7 

2.8 

6.30 

0.153600  »-' 

.30 

.38 

8.6 

+  19000 

5.1 

8.0 

0.053 

0.000715 

.25 

.26 

12.6 

-  11000 

6.6 

8.6 

0.030 

0.000189 

.28 

.31 

10.5 

+  17000 

4.6 

7.0 

0.13 

0.001230 

26 

11.3 

0.0025 

.29 

.29 

11.2 

-  3000 

7.1 

9.4 

0.014 

0.000111 

17 

9.9 

0.009 

.22 

.22 

14.8 

-  7000 

8.2 

9.9 

0.009 

0.000041 

278  CLIMATIC  CHANGES 

be  made  for  disturbances  by  outside  bodies  such  as  com- 
panion stars.  Hence  the  inclusion  of  column  14. 

On  the  basis  of  column  14  and  of  the  movements  and 
distances  of  the  stars  as  given  in  the  other  columns  Fig. 
10  has  been  prepared.  This  gives  an  estimate  of  the 
approximate  electrical  energy  received  by  the  sun  from 
the  nearest  stars  for  70,000  years  before  and  after  the 
present.  It  is  based  on  the  twenty-six  stars  for  which 
complete  data  are  available  in  Table  6.  The  inclusion  of 
the  other  twelve  would  not  alter  the  form  of  the  curve, 
for  even  the  largest  of  them  would  not  change  any  part 
by  more  than  about  half  of  1  per  cent,  if  as  much. 
Nor  would  the  curve  be  visibly  altered  by  the  omission 
of  all  except  four  of  the  twenty-six  stars  actually  used. 
The  four  that  are  important,  and  their  relative  lumi- 
nosity when  nearest  the  sun,  are  Sirius  429,000,  Altair 
153,000,  Alpha  Centauri  117,500,  and  Procyon  51,300. 
The  figure  for  the  next  star  is  only  4970,  while  for  this 
star  combined  with  the  other  twenty-one  that  are  unim- 
portant it  is  only  24,850. 

Figure  10  is  not  carried  more  than  70,000  years  into 
the  past  or  into  the  future  because  the  stars  near  the 
sun  at  more  remote  times  are  not  included  among  the 
thirty-eight  having  the  largest  known  parallaxes.  That 
is,  they  have  either  moved  away  or  are  not  yet  near 
enough  to  be  included.  Indeed,  as  Dr.  Schlesinger 
strongly  emphasizes,  there  may  be  swiftly  moving,  bright 
or  gigantic  stars  which  are  now  quite  far  away,  but  whose 
inclusion  would  alter  Fig.  10  even  within  the  limits  of 
the  140,000  years  there  shown.  It  is  almost  certain,  how- 
ever, that  the  most  that  these  would  do  would  be  to  raise, 
but  not  obliterate,  the  minima  on  either  side  of  the  main 
maximum. 

In  preparing  Fig.  10  it  has  been  necessary  to  make 


1 

3     o 


o 


280  CLIMATIC  CHANGES 

allowance  for  double  stars.  Passing  by  the  twenty-two 
unimportant  stars,  it  appears  that  the  companion  of 
Sirius  is  eight  or  ten  magnitudes  smaller  than  that  star, 
while  the  companions  of  Procyon  and  Altair  are  five  or 
more  magnitudes  smaller  than  their  bright  comrades. 
This  means  that  the  luminosity  of  the  faint  components 
is  at  most  only  1  per  cent  of  that  of  their  bright  com- 
panions and  in  the  case  of  Sirius  not  a  hundredth  of  1 
per  cent.  Hence  their  inclusion  would  have  no  visible 
effect  on  Fig.  10.  In  Alpha  Centauri,  on  the  other  hand, 
the  two  components  are  of  almost  the  same  magnitude. 
For  this  reason  the  effective  radiation  of  that  star  as 
given  in  column  14  is  doubled  in  Fig.  10,  while  for 
another  reason  it  is  raised  still  more.  The  other  reason 
is  that  if  our  inferences  as  to  the  electrical  effect  of  the 
sun  on  the  earth  and  of  the  planets  on  the  sun  are  cor- 
rect, double  stars,  as  we  have  seen,  must  be  much  more 
effective  electrically  than  single  stars.  By  the  same 
reasoning  two  bright  stars  close  together  must  excite 
one  another  much  more  than  a  bright  star  and  a  very 
faint  one,  even  if  the  distances  in  both  cases  are  the  same. 
So,  too,  other  things  being  equal,  a  triple  star  must  be 
more  excited  electrically  than  a  double  star.  Hence  in 
preparing  Fig.  10  all  double  stars  receive  double  weight 
and  each  part  of  Alpha  Centauri  receives  an  additional 
50  per  cent  because  both  parts  are  bright  and  because 
they  have  a  third  companion  to  help  in  exciting  them. 

According  to  the  electro-stellar  hypothesis,  Alpha  Cen- 
tauri is  more  important  climatically  than  any  other  star 
in  the  heavens  not  only  because  it  is  triple  and  bright,  but 
because  it  is  the  nearest  of  all  stars,  and  moves  fairly 
rapidly.  Sirius  and  Procyon  move  slowly  in  respect  to 
the  sun,  only  about  eleven  and  eight  kilometers  per 
second  respectively,  and  their  distances  at  minimum  are 


THE  SUN'S  JOURNEY  THROUGH  SPACE      281 

fairly  large,  that  is,  8  and  10.2  light  years.  Hence  their 
effect  on  the  sun  changes  slowly.  Altair  moves  faster, 
about  twenty-six  kilometers  per  second,  and  its  minimum 
distance  is  6.4  light  years,  so  that  its  effect  changes  fairly 
rapidly.  Alpha  Centauri  moves  about  twenty-four  kilo- 
meters per  second,  and  its  minimum  distance  is  only  3.2 
light  years.  Hence  its  effect  changes  very  rapidly,  the 
change  in  its  apparent  luminosity  as  seen  from  the  sun 
amounting  at  maximum  to  about  30  per  cent  in  10,000 
years  against  14  per  cent  for  Altair,  4  for  Sirius,  and  2 
for  Procyon.  The  vast  majority  of  the  stars  change  so 
much  more  slowly  than  even  Procyon  that  their  effect  is 
almost  uniform.  All  the  stars  at  a  distance  of  more  than 
perhaps  twenty  or  thirty  light  years  may  be  regarded  as 
sending  to  the  sun  a  practically  unchanging  amount  of 
radiation.  It  is  the  bright  stars  within  this  limit  which 
are  important,  and  their  importance  increases  with  their 
proximity,  their  speed  of  motion,  and  the  brightness  and 
number  of  their  companions.  Hence  Alpha  Centauri 
causes  the  main  maximum  in  Fig.  10,  while  Sirius,  Altair, 
and  Procyon  combine  to  cause  a  general  rise  of  the  curve 
from  the  past  to  the  future. 

Let  us  now  interpret  Fig.  10  geologically.  The  low  posi- 
tion of  the  curve  fifty  to  seventy  thousand  years  ago 
suggests  a  mild  inter-glacial  climate  distinctly  less  severe 
than  that  of  the  present.  Geologists  say  that  such  was  the 
case.  The  curve  suggests  a  glacial  epoch  culminating 
about  28,000  years  ago.  The  best  authorities  put  the  cli- 
max of  the  last  glacial  epoch  between  twenty-five  and 
thirty  thousand  years  ago.  The  curve  shows  an  ameliora- 
tion of  climate  since  that  time,  although  it  suggests  that 
there  is  still  considerable  severity.  The  retreat  of  the  ice 
from  North  America  and  Europe,  and  its  persistence  in 
Greenland  and  Antarctica  agree  with  this.  And  the  curve 


282  CLIMATIC  CHANGES 

indicates  that  the  change  of  climate  is  still  persisting,  a 
conclusion  in  harmony  with  the  evidence  as  to  historic 
changes. 

If  Alpha  Centauri  is  really  so  important,  the  effect  of 
its  variations,  provided  it  has  any,  ought  perhaps  to  be 
evident  in  the  sun.  The  activity  of  the  star's  atmosphere 
presumably  varies,  for  the  orbits  of  the  two  components 
have  an  eccentricity  of  0.51.  Hence  during  their  period 
of  revolution,  81.2  years,  the  distance  between  them 
ranges  from  1,100,000,000  to  3,300,000,000  miles.  They 
were  at  a  minimum  distance  in  1388,  1459,  1550,  1631, 
1713,  1794,  1875,  and  will  be  again  in  1956.  In  Fig. 
11,  showing  sunspot  variations,  it  is  noticeable  that  the 
years  1794  and  1875  come  just  at  the  ends  of  periods  of 
unusual  solar  activity,  as  indicated  by  the  heavy  hori- 
zontal line.  A  similar  period  of  great  activity  seems  to 
have  begun  about  1914.  If  its  duration  equals  the  average 
of  its  two  predecessors,  it  will  end  about  1950.  Back  in 
the  fourteenth  century  a  period  of  excessive  solar  ac- 
tivity, which  has  already  been  described,  culminated  from 
1370  to  1385,  or  just  before  the  two  parts  of  Alpha  Cen- 
tauri were  at  a  minimum  distance.  Thus  in  three  and 
perhaps  four  cases  the  sun  has  been  unusually  active 
during  a  time  when  the  two  parts  of  the  star  were  most 
rapidly  approaching  each  other  and  when  their  atmos- 
pheres were  presumably  most  disturbed  and  their  elec- 
trical emanations  strongest. 

The  fact  that  Alpha  Centauri,  the  star  which  would  be 
expected  most  strongly  to  influence  the  sun,  and  hence 
the  earth,  was  nearest  the  sun  at  the  climax  of  the  last 
glacial  epoch,  and  that  today  the  solar  atmosphere  is 
most  active  when  the  star  is  presumably  most  disturbed 
may  be  of  no  significance.  It  is  given  for  what  it  is  worth. 
Its  importance  lies  not  in  the  fact  that  it  proves  any- 


284  CLIMATIC  CHANGES 

thing,  but  that  no  contradiction  is  found  when  we  test 
the  electro-stellar  hypothesis  by  facts  which  were  not 
thought  of  when  the  hypothesis  was  framed.  A  vast 
amount  of  astronomical  work  is  still  needed  before  the 
matter  can  be  brought  to  any  definite  conclusion.  In  case 
the  hypothesis  stands  firm,  it  may  be  possible  to  use  the 
stars  as  a  help  in  determining  the  exact  chronology  of  the 
later  part  of  geological  times.  If  the  hypothesis  is  dis- 
proved, it  will  merely  leave  the  question  of  solar  varia- 
tions where  it  is  today.  It  will  not  influence  the  main 
conclusions  of  this  book  as  to  the  causes  and  nature  of 
climatic  changes.  Its  value  lies  in  the  fact  that  it  calls 
attention  to  new  lines  of  research. 


CHAPTER  XVI 
THE  EARTH'S  CRUST  AND  THE  SUN 

A  LTHOUGH  the  problems  of  this  book  may  lead  far 
/%  afield,  they  ultimately  bring  us  back  to  the  earth 
1  %  and  to  the  present.  Several  times  in  the  preceding 
pages  there  has  been  mention  of  the  fact  that  periods  of 
extreme  climatic  fluctuations  are  closely  associated  with 
great  movements  of  the  earth's  crust  whereby  mountains 
are  uplifted  and  continents  upheaved.  In  attempting  to 
explain  this  association  the  general  tendency  has  been 
to  look  largely  at  the  past  instead  of  the  present.  Hence 
it  has  been  almost  impossible  to  choose  among  three 
possibilities,  all  beset  with  difficulties.  First,  the  move- 
ments of  the  crust  may  have  caused  the  climatic  fluctua- 
tions ;  second,  climatic  changes  may  cause  crustal  move- 
ments ;  and  third,  variations  in  solar  activity  or  in  some 
other  outside  agency  may  give  rise  to  both  types  of  terres- 
trial phenomena. 

The  idea  that  movements  of  the  earth's  crust  are  the 
main  cause  of  geological  changes  of  climate  is  becoming 
increasingly  untenable  as  the  complexity  and  rapidity  of 
climatic  changes  become  more  clear,  especially  during 
post-glacial  times.  It  implies  that  the  earth's  surface 
moves  up  and  down  with  a  speed  and  facility  which 
appear  to  be  out  of  the  question.  If  volcanic  activity  be 
invoked  the  problem  becomes  no  clearer.  Even  if  volcanic 
dust  should  fill  the  air  frequently  and  completely,  neither 
its  presence  nor  absence  would  produce  such  peculiar  fea- 


286  CLIMATIC  CHANGES 

tures  as  the  localization  of  glaciers,  the  distribution  of 
loess,  and  the  mild  climate  of  most  parts  of  geological 
time.  Nevertheless,  because  of  the  great  difficulties  pre- 
sented by  the  other  two  possibilities  many  geologists 
still  hold  that  directly  or  indirectly  the  greater  climatic 
changes  have  been  mainly  due  to  movements  of  the 
earth 's  crust  and  to  the  reaction  of  the  crustal  movements 
on  the  atmosphere. 

The  possibility  that  climatic  changes  are  in  themselves 
a  cause  of  movements  of  the  earth's  crust  seems  so  im- 
probable that  no  one  appears  to  have  investigated  it  with 
any  seriousness.  Nevertheless,  it  is  worth  while  to  raise 
the  question  whether  climatic  extremes  may  cooperate 
with  other  agencies  in  setting  the  time  when  the  earth's 
crust  shall  be  deformed. 

As  to  the  third  possibility,  it  is  perfectly  logical  to 
ascribe  both  climatic  changes  and  crustal  deformation  to 
some  outside  agency,  solar  or  otherwise,  but  hitherto 
there  has  been  so  little  evidence  on  this  point  that  such 
an  ascription  has  merely  begged  the  question.  If  heavenly 
bodies  should  approach  the  earth  closely  enough  so  that 
their  gravitational  stresses  caused  crustal  deformation, 
all  life  would  presumably  be  destroyed.  As  to  the  sun, 
there  has  hitherto  been  no  conclusive  evidence  that  it  is 
related  to  crustal  movements,  although  various  writers 
have  made  suggestions  along  this  line.  In  this  chapter 
we  shall  carry  these  suggestions  further  and  shall  see 
that  they  are  at  least  worthy  of  study. 

As  a  preliminary  to  this  study  it  may  be  well  to  note 
that  the  coincidence  between  movements  of  the  earth's 
crust  and  climatic  changes  is  not  so  absolute  as  is  some- 
times supposed.  For  example,  the  profound  crustal 
changes  at  the  end  of  the  Mesozoic  were  not  accompanied 
by  widespread  glaciation  so  far  as  is  yet  known,  although 


THE  EARTH'S  CRUST  AND  THE  SUN        287 

the  temperature  appears  to  have  been  lowered.  Nor  was 
the  violent  volcanic  and  diastrophic  activity  in  the  Mio- 
cene associated  with  extreme  climates.  Indeed,  there 
appears  to  have  been  little  contrast  from  zone  to  zone, 
for  figs,  bread  fruit  trees,  tree  ferns,  and  other  plants  of 
low  latitudes  grew  in  Greenland.  Nevertheless,  both  at 
the  end  of  the  Mesozoic  and  in  the  Miocene  the  climate 
may  possibly  have  been  severe  for  a  time,  although  the 
record  is  lost.  On  the  other  hand,  Kirk's  recent  discovery 
of  glacial  till  in  Alaska  between  beds  carrying  an  un- 
doubted Middle  Silurian  fauna  indicates  glaciation  at  a 
time  when  there  was  little  movement  of  the  crust  so  far 
as  yet  appears.1  Thus  we  conclude  that  while  climatic 
changes  and  crustal  movements  usually  occur  together, 
they  may  occur  separately. 

According  to  the  solar-cyclonic  hypothesis  such  a  con- 
dition is  to  be  expected.  If  the  sun  were  especially  active 
when  the  terrestrial  conditions  prohibited  glaciation, 
changes  of  climate  would  still  occur,  but  they  would  be 
milder  than  under  other  circumstances,  and  would  leave 
little  record  in  the  rocks.  Or  there  might  be  glaciation  in 
high  latitudes,  such  as  that  of  southern  Alaska  in  the 
Middle  Silurian,  and  none  elsewhere.  On  the  other  hand, 
when  the  sun  was  so  inactive  that  no  great  storminess 
occurred,  the  upheaval  of  continents  and  the  building  of 
mountains  might  go  on  without  the  formation  of  ice 
sheets,  as  apparently  happened  at  the  end  of  the  Meso- 
zoic. The  lack  of  absolute  coincidence  between  glaciation 
and  periods  of  widespread  emergence  of  the  lands  is 
evident  even  today,  for  there  is  no  reason  to  suppose 
that  the  lands  are  notably  lower  or  less  extensive  now 
than  they  were  during  the  Pleistocene  glaciation.  In 
fact,  there  is  much  evidence  that  many  areas  have  risen 

iE.  Kirk:  Paleozoic  Glaciation  in  Alaska;  Am.  Jour.  Sci.,  1918,  p.  511. 


288  CLIMATIC  CHANGES 

since  that  time.  Yet  glaciation  is  now  far  less  extensive 
than  in  the  Pleistocene.  Any  attempt  to  explain  this  dif- 
ference on  the  basis  of  terrestrial  changes  is  extremely 
difficult,  for  the  shape  and  altitude  of  continents  and 
mountains  have  not  changed  much  in  twenty  or  thirty 
thousand  years.  Yet  the  present  moderately  mild  epoch, 
like  the  puzzling  inter-glacial  epochs  of  earlier  times,  is 
easily  explicable  on  the  assumption  that  the  sun 's  atmos- 
phere may  sometimes  vary  in  harmony  with  crustal 
activity,  but  does  not  necessarily  do  so  at  all  times. 

Turning  now  to  the  main  problem  of  how  climatic 
changes  may  be  connected  with  movements  of  the  earth's 
crust,  let  us  follow  our  usual  method  and  examine  what 
is  happening  today.  Let  us  first  inquire  whether  earth- 
quakes, which  are  one  of  the  chief  evidences  that  crustal 
movements  are  actually  taking  place  in  our  own  times, 
show  any  connection  with  sunspots.  In  order  to  test  this, 
we  have  compared  Milne's  Catalogue  of  Destructive 
Earthquakes  from  1800  to  1899,  with  Wolf's  sunspot 
numbers  for  the  same  period  month  by  month.  The  earth- 
quake catalogue,  as  its  compiler  describes  it,  "is  an 
attempt  to  give  a  list  of  earthquakes  which  have  an- 
nounced changes  of  geological  importance  in  the  earth's 
crust;  movements  which  have  probably  resulted  in  the 
creation  or  the  extension  of  a  line  of  fault,  the  vibrations 
accompanying  which  could,  with  proper  instruments, 
have  been  recorded  over  a  continent  or  the  whole  surface 
of  our  world.  Small  earthquakes  have  been  excluded, 
while  the  number  of  large  earthquakes  both  for  ancient 
and  modern  times  has  been  extended.  As  an  illustration 
of  exclusion,  I  may  mention  that  between  1800  and  1808, 
which  are  years  taken  at  random,  I  find  in  Mallet 's  cata- 
logue 407  entries.  Only  thirty-seven  of  these,  which  were 
accompanied  by  structural  damage,  have  been  retained. 


THE  EARTH'S  CRUST  AND  THE  SUN 


289 


Other  catalogues  such  as  those  of  Perry  and  Fuchs  have 
been  treated  similarly. '  '2 

If  the  earthquakes  in  such  a  carefully  selected  list  bear 
a  distinct  relation  to  sunspots,  it  is  at  least  possible  and 
perhaps  probable  that  a  similar  relation  may  exist  be- 
tween solar  activity  and  geological  changes  in  the  earth 's 
crust.  The  result  of  the  comparison  of  earthquakes  and 
sunspots  is  shown  in  Table  7.  The  first  column  gives  the 
sunspot  numbers ;  the  second,  the  number  of  months  that 
had  the  respective  spot  numbers  during  the  century  from 
1800  to  1899.  Column  C  shows  the  total  number  of  earth- 
quakes during  the  months  having  any  particular  degree 
of  spottedness ;  while  D,  which  is  the  significant  column, 
gives  the  average  number  of  destructive  earthquakes  per 
month  under  each  of  the  six  conditions  of  solar  spotted- 


DESTRUCTIVE 

TABLE  7 
EARTHQUAKES 

FROM  1800  TO 

1899  COMPARED 

WITH  SUNSPOTS 

A 

B 

c 

D 

E 

F 

Average 

Average 

Number 

number 

Number 

number 

of  earth- 

of earth- 

of months 

Number 

of  earth- 

quakes in 

quakes  in 

Sunspot 

per  Wolf  's 

of  earth- 

quakes per 

succeeding 

succeeding 

numbers 

Table 

quakes 

month 

month 

month 

0-  15 

344 

522 

1.52 

512 

1.49 

15-  30 

]94 

306 

1.58 

310 

1.60 

30-  50 

237 

433 

•1.83 

439 

1.85 

50-  70 

195 

402 

2.06 

390 

2.00 

70-100 

135 

286 

'     2.12 

310 

2.30 

over  100 

95 

218 

2.30 

175 

1.84 

2  J.  Milne:  Catalogue  of  Destructive  Earthquakes;  Eep.  Brit.  Asso.  Adv. 
Sci.,  1911. 


290  CLIMATIC  CHANGES 

ness.  The  regularity  of  column  D  is  so  great  as  to  make 
it  almost  certain  that  we  are  here  dealing  with  a  real 
relationship.  Column  F,  which  shows  the  average  number 
of  earthquakes  in  the  month  succeeding  any  given  condi- 
tion of  the  sun,  is  still  more  regular  except  for  the  last 
entry. 

The  chance  that  six  numbers  taken  at  random  will 
arrange  themselves  in  any  given  order  is  one  in  720.  In 
other  words,  there  is  one  chance  in  720  that  the  regularity 
of  column  D  is  accidental.  But  column  F  is  as  regular  as 
column  D  except  for  the  last  entry.  If  columns  D  and  E 
were  independent  there  would  be  one  chance  in  about 
500,000  that  the  six  numbers  in  both  columns  would 
fall  in  the  same  order,  and  one  chance  in  14,400  that 
five  numbers  in  each  would  fall  in  the  same  order. 
But  the  two  columns  are  somewhat  related,  for  although 
the  after-shocks  of  a  great  earthquake  are  never  included 
in  Milne's  table,  a  world-shaking  earthquake  in  one 
region  during  a  given  month  probably  creates  conditions 
that  favor  similar  earthquakes  elsewhere  during  the  next 
month.  Hence  the  probability  that  we  are  dealing  with  a 
purely  accidental  arrangement  in  Table  7  is  less  than  one 
in  14,400  and  greater  than  one  in  500,000.  It  may  be  one 
in  20,000  or  100,000.  In  any  event  it  is  so  slight  that  there 
is  high  probability  that  directly  or  indirectly  sunspots 
and  earthquakes  are  somehow  connected. 

In  ascertaining  the  relation  between  sunspots  and 
earthquakes  it  would  be  well  if  we  could  employ  the  strict 
method  of  correlation  coefficients.  This,  however,  is  im- 
possible for  the  entire  century,  for  the  record  is  by  no 
means  homogeneous.  The  earlier  decades  are  represented 
by  only  about  one-fourth  as  many  earthquakes  as  the 
later  ones,  a  condition  which  is  presumably  due  to  lack  of 
information.  This  makes  no  difference  with  the  method 


THE  EARTH'S  CRUST  AND  THE  SUN    291 

employed  in  Table  7,  since  years  with  many  and  few  sun- 
spots  are  distributed  almost  equally  throughout  the 
entire  nineteenth  century,  but  it  renders  the  method  of 
correlation  coefficients  inapplicable.  During  the  period 
from  1850  onward  the  record  is  much  more  nearly  homo- 
geneous, though  not  completely  so.  Even  in  these  later 
decades,  however,  allowance  must  be  made  for  the  fact 
that  there  are  more  earthquakes  in  winter  than  in 
summer,  the  average  number  per  month  for  the  fifty 
years  being  as  follows : 

Jan.  2.8  May  2.4  Sept.  2.5 

Feb.  2.4  June  2.3  Oct.  2.6 

Mar.  2.5  July  2.4  Nov.  2.7 

Apr.  2.4  Aug.  2.4  Dec.  2.8 

The  correlation  coefficient  between  the  departures  from 
these  monthly  averages  and  the  corresponding  depar- 
tures from  the  monthly  averages  of  the  sunspots  for  the 
same  period,  1850-1899,  are  as  follows : 

Sunspots  and  earthquakes  of  same  month:  -fO.042,  or  1.5 
times  the  probable  error. 

Sunspots  of  a  given  month  and  earthquakes  of  that  month 
and  the  next :  -f-0.084,  or  3.1  times  the  probable  error. 

Sunspots  of  three  consecutive  months  and  earthquakes  of 
three  consecutive  months  allowing  a  lag  of  one  month,  i.e.,  sun- 
spots  of  January,  February,  and  March  compared  with  earth- 
quakes of  February,  March,  and  April;  sunspots  of  February, 
March,  and  April  with  earthquakes  of  March,  April,  and  May, 
etc. ;  +0.112,  or  4.1  times  the  probable  error. 

These  coefficients  are  all  small,  but  the  number  of  in- 
dividual cases,  600  months,  is  so  large  that  the  probable 
error  is  greatly  reduced,  being  only  ±0.027  or  ±0.028. 
Moreover,  the  nature  of  our  data  is  such  that  even  if 


292  CLIMATIC  CHANGES 

there  is  a  strong  connection  between  solar  changes  and 
earth  movements,  we  should  not  expect  a  large  correla- 
tion coefficient.  In  the  first  place,  as  already  mentioned, 
the  earthquake  data  are  not  strictly  homogeneous. 
Second,  an  average  of  about  two  and  one-half  strong 
earthquakes  per  month  is  at  best  only  a  most  imperfect 
indication  of  the  actual  movement  of  the  earth's  crust. 
Third,  the  sunspots  are  only  a  partial  and  imperfect 
measure  of  the  activity  of  the  sun's  atmosphere.  Fourth, 
the  relation  between  solar  activity  and  earthquakes  is 
almost  certainly  indirect.  In  view  of  all  these  conditions, 
the  regularity  of  Table  7  and  the  fact  that  the  most  im- 
portant correlation  coefficient  rises  to  more  than  four 
times  the  probable  error  makes  it  almost  certain  that  the 
solar  and  terrestrial  phenomena  are  really  connected. 

We  are  now  confronted  by  the  perplexing  question  of 
how  this  connection  can  take  place.  Thus  far  only  three 
possibilities  present  themselves,  and  each  is  open  to 
objections.  The  chief  agencies  concerned  in  these  three 
possibilities  are  heat,  electricity,  and  atmospheric  pres- 
sure. Heat  may  be  dismissed  very  briefly.  We  have  seen 
that  the  earth's  surface  becomes  relatively  cool  when 
the  sun  is  active.  Theoretically  even  the  slightest  change 
in  the  temperature  of  the  earth's  surface  must  influence 
the  thermal  gradient  far  into  the  interior  and  hence  cause 
a  change  of  volume  which  might  cause  movements  of  the 
crust.  Practically  the  heat  of  the  surface  ceases  to  be  of 
appreciable  importance  at  a  depth  of  perhaps  twenty 
feet,  and  even  at  that  depth  it  does  not  act  quickly  enough 
to  cause  the  relatively  prompt  response  which  seems  to  be 
characteristic  of  earthquakes  in  respect  to  the  sun. 

The  second  possibility  is  based  on  the  relationship 
between  solar  and  terrestrial  electricity.  When  the  sun 
is  active  the  earth's  atmospheric  electrical  potential  is 


THE  EARTH'S  CRUST  AND  THE  SUN    293 

subject  to  slight  variations.  It  is  well  known  that  when 
two  opposing  points  of  an  ionized  solution  are  oppositely 
charged  electrically,  a  current  passes  through  the  liquid 
and  sets  up  electrolysis  whereby  there  is  a  segregation 
of  materials,  and  a  consequent  change  in  the  volume  of 
the  parts  near  the  respective  electrical  poles.  The  same 
process  takes  place,  although  less  freely,  in  a  hot  mass 
such  as  forms  the  interior  of  the  earth.  The  question 
arises  whether  internal  electrical  currents  may  not  pass 
between  the  two  oppositely  charged  poles  of  the  earth, 
or  even  between  the  great  continental  masses  and  the 
regions  of  heavier  rock  which  underlie  the  oceans.  Could 
this  lead  to  electrolysis,  hence  to  differentiation  in  vol- 
ume, and  thus  to  movements  of  the  earth's  crust?  Could 
the  results  vary  in  harmony  with  the  sun!  Bowie3  has 
shown  that  numerous  measurements  of  the  strength  and 
direction  of  the  earth's  gravitative  pull  are  explicable 
only  on  the  assumption  that  the  upheaval  of  a  continent 
or  a  mountain  range  is  due  in  part  not  merely  to  pres- 
sure, or  even  to  flowage  of  the  rocks  beneath  the  crust, 
but  also  to  an  actual  change  in  volume  whereby  the  rocks 
beneath  the  continent  attain  relatively  great  volume  and 
those  under  the  oceans  a  small  volume  in  proportion  to 
their  weight.  The  query  arises  whether  this  change  of 
volume  may  be  related  to  electrical  currents  at  some 
depth  below  the  earth's  surface. 

The  objections  to  this  hypothesis  are  numerous.  First, 
there  is  little  evidence  of  electrolytic  differentiation  in 
the  rocks.  Second,  the  outer  part  of  the  earth's  crust  is  a 
very  poor  conductor  so  that  it  is  doubtful  whether  even 
a  high  degree  of  electrification  of  the  surface  would  have 
much  effect  on  the  interior.  Third,  electrolysis  due  to  any 

3  Wm.  Bowie:  Lecture  before  the  Geological  Club  of  Yale  University. 
See  Am.  Jour.  Sci.,  1921. 


294  CLIMATIC  CHANGES 

such  mild  causes  as  we  have  here  postulated  must  be  an 
extremely  slow  process,  too  slow,  presumably,  to  have 
any  appreciable  result  within  a  month  or  two.  Other 
objections  join  with  these  three  in  making  it  seem  im- 
probable that  the  sun's  electrical  activity  has  any  direct 
effect  upon  movements  of  the  earth's  crust. 

The  third,  or  meteorological  hypothesis,  which  makes 
barometric  pressure  the  main  intermediary  between  solar 
activity  and  earthquakes,  seems  at  first  sight  almost  as 
improbable  as  the  thermal  and  electrical  hypotheses. 
Nevertheless,  it  has  a  certain  degree  of  observational 
support  of  a  kind  which  is  wholly  lacking  in  the  other  two 
cases.  Among  the  extensive  writings  on  the  periodicity  of 
earthquakes  one  main  fact  stands  out  with  great  dis- 
tinctness :  earthquakes  vary  in  number  according  to  the 
season.  This  fact  has  already  been  shown  incidentally  in 
the  table  of  earthquake  frequency  by  months.  If  allow- 
ance is  made  for  the  fact  that  February  is  a  short  month, 
there  is  a  regular  decrease  in  the  frequency  of  severe 
earthquakes  from  December  and  January  to  June.  Since 
most  of  Milne's  earthquakes  occurred  in  the  northern 
hemisphere,  this  means  that  severe  earthquakes  occur  in 
winter  about  20  per  cent  oftener  than  in  summer. 

The  most  thorough  investigation  of  this  subject  seems 
to  have  been  that  of  Davisson.4  His  results  have  been 
worked  over  and  amplified  by  Knott,5  who  has  tested 
them  by  Schuster 's  exact  mathematical  methods.  His  re- 
sults are  given  in  Table  8.6  Here  the  northern  hemisphere 

4  Chas.  Davisson:  On  the  Annual  and  Semi-annual  Seismic  Periods; 
Boy.  Soc.  of  London,  Philosophical  Transactions,  Vol.  184,  1893,  1107  ff. 

s  C.  G.  Knott :  The  Physics  of  Earthquake  Phenomena,  Oxford,  1908. 

« In  Table  8  the  first  column  indicates  the  region ;  the  second,  the  dates ; 
and  the  third,  the  number  of  shocks.  The  fourth  column  gives  the  month  in 
which  the  annual  maximum  occurs  when  the  crude  figures  are  smoothed  by 
the  use  of  overlapping  six-monthly  means.  In  other  words,  the  average  for 
each  successive  six  months  has  been  placed  in  the  middle  of  the  period. 


THE  EARTH'S  CRUST  AND  THE  SUN 


295 


TABLE  8 
SEASONAL  MARCH  OF  EARTHQUAKES 

AFTER  DAVISSON 

AND  KNOTT 

A 

B 

C 

D 

E 

F 

G 

b 

a 

I 

1 

"8? 

V.-2-8 

1 

Region 

v| 

§  § 

£ 

"S  15 

111 

2 

J  J 

1* 

II 

1 

II 

III 

| 

Northern  Hemisphere 

223-1850 

5879 

Dec. 

0.110 

0.023 

4.8 

Northern  Hemisphere 

1865-1884 

8133 

Dec. 

0.290 

0.020 

14.5 

Europe 

1865-1884 

5499 

Dec. 

0.350 

0.024 

14.6 

Europe 

306-1843 

1961 

Dec. 

0.220 

0.040 

5.5 

Southeast  Europe 

1859-1887 

3470 

Dec. 

0.210 

0.030 

7.0 

Vesuvius  District 

1865-1883 

513 

Dec. 

0.250 

0.078 

3.2 

Italy: 

Old  Tromometre 

1872-1887 

61732 

Dec. 

0.490 

0.007 

70.0 

Old  Tromometre 

1876-1887 

38546 

Dec. 

0.460 

0.009 

49.5 

Normal  Tromometre 

1876-1887 

38546 

Dec. 

0.490 

0.009 

52.8 

Balkan,  etc. 

1865-1884 

624 

Dec. 

0.270 

0.071 

3.8 

Hungary,  etc. 

1865-1884 

384 

Dec. 

0.310 

0.090 

3.4 

Italy 

1865-1883 

2350 

Dec.  (Sept.) 

0.140 

0.037 

3.8 

Grecian  Archip. 

1859-1881 

3578 

Dec.-Jan. 

0.164 

0.030 

5.5 

Austria 

1865-1884 

461 

Jan. 

0.370 

0.083 

4.4 

Switzerland,  etc. 

1865-1883 

524 

Jan. 

0.560 

0.077 

7.3 

Asia 

1865-1884 

458 

Feb. 

0.330 

0.083 

4.0 

North  America 

1865-1884 

552 

Nov. 

0.350 

0.075 

4.7 

California 

1850-1886 

949 

Oct. 

0.300 

0.058 

5.2 

Japan 

1878-1881 

246 

Dec. 

0.460 

0.113 

4.1 

Japan 

1872-1880 

367 

Dec.-Jan. 

0.256 

0.093 

2.8 

Japan 

1876-1891 

1104 

Feb. 

0.190 

0.053 

3.6 

Japan 

1885-1889 

2997 

Oct. 

0.080 

0.032 

2.5 

Zante 

1825-1863 

1326 

Aug. 

0.100 

0.049 

2.0 

Italy,  North  of  Naples 

1865-1883 

1513 

Sept.  (Nov.) 

0.210 

0.046 

4.6 

East  Indies 

1873-1881 

515 

Aug.,  Oct., 

0.071? 

0.078 

0.9 

or  Dec.  f 

Malay  Archip. 

1865-1884 

598 

May 

0.190 

0.072 

2.6 

New  Zealand 

1869-1879 

585 

Aug.-Sept. 

0.203 

0.073 

2.8 

Chile 

1873-1881 

212 

July 

0.480 

0.122 

3.9 

Southern  Hemisphere 

1865-1884 

751 

July 

0.370 

0.065 

5.7 

New  Zealand 

1868-1890 

641 

March,  May 

0.050 

0.070 

0.7 

Chile 

1865-1883? 

316 

July,  Dec. 

0.270 

0.100 

2.7 

Peru,  Bolivia 

1865-1884 

350 

July 

0.480 

0.095 

5.1 

296  CLIMATIC  CHANGES 

is  placed  first ;  then  come  the  East  Indies  and  the  Malay 
Archipelago  lying  close  to  the  equator;  and  finally  the 
southern  hemisphere.  In  the  northern  hemisphere  prac- 
tically all  the  maxima  come  in  the  winter,  for  the  month 
of  December  appears  in  fifteen  cases  out  of  the  twenty- 
five  in  column  D,  while  January,  February,  or  November 
appears  in  six  others.  It  is  also  noticeable  that  in  sixteen 
cases  out  of  twenty-five  the  ratio  of  the  actual  to  the  ex- 
pected amplitude  in  column  G  is  four  or  more,  so  that  a 
real  relationship  is  indicated,  while  the  ratio  falls  below 
three  only  in  Japan  and  Zante.  The  equatorial  data, 
unlike  those  of  the  northern  hemisphere,  are  indefinite, 
for  in  the  East  Indies  no  month  shows  a  marked  maxi- 
mum and  the  expected  amplitude  exceeds  the  actual  am- 
plitude. Even  in  the  Malay  Archipelago,  which  shows  a 
maximum  in  May,  the  ratio  of  actual  to  expected  ampli- 
tude is  only  2.6.  Turning  to  the  southern  hemisphere,  the 
winter  months  of  that  hemisphere  are  as  strongly  marked 
by  a  maximum  as  are  the  winter  months  of  the  northern 

Thus  the  average  of  January  to  June,  inclusive,  is  placed  between  March 
and  April,  that  for  February  to  July  between  April  and  May,  and  so  on. 
This  method  eliminates  the  minor  fluctuations  and  also  all  periodicities 
having  a  duration  of  less  than  a  year.  If  there  were  no  annual  periodicity 
the  smoothing  would  result  in  practically  the  same  figure  for  each  month. 
The  column  marked  "Amplitude"  gives  the  range  from  the  highest  month 
to  the  lowest  divided  by  the  number  of  earthquakes  and  then  corrected 
according  to  Schuster's  method  which  is  well  known  to  mathematicians, 
but  which  is  so  confusing  to  the  layman  that  it  will  not  be  described.  Next, 
in  the  column  marked  "Expected  Amplitude,"  we  have  the  amplitude  that 
would  be  expected  if  a  series  of  numbers  corresponding  to  the  earthquake 
numbers  and  having  a  similar  range  were  arranged  in  accidental  order 
throughout  the  year.  This  also  is  calculated  by  Schuster's  method  in  which 
the  expected  amplitude  is  equal  to  the  square  root  of  "pi"  divided  by  the 
number  of  shocks.  When  the  actual  amplitude  is  four  or  more  times  the 
expected  amplitude,  the  probability  that  there  is  a  real  periodicity  in  the 
observed  phenomena  becomes  so  great  that  we  may  regard  it  as  practically 
certain.  If  there  is  no  periodicity  the  two  are  equal.  The  last  column  gives 
the  number  of  times  by  which  the  actual  exceeds  the  expected  amplitude, 
and  thus  is  a  measure  of  the  probability  that  earthquakes  vary  system- 
atically in  a  period  of  a  year. 


THE  EARTH'S  CRUST  AND  THE  SUN        297 

hemisphere.  July  or  August  appears  in  five  out  of  six 
cases.  Here  the  ratio  between  the  actual  and  expected 
amplitudes  is  not  so  great  as  in  the  northern  hemisphere. 
Nevertheless,  it  is  practically  four  in  Chile,  and  exceeds 
five  in  Peru  and  Bolivia,  and  in  the  data  for  the  entire 
southern  hemisphere. 

The  whole  relationship  between  earthquakes  and  the 
seasons  in  the  northern  and  southern  hemispheres  is 
summed  up  in  Fig.  12  taken  from  Knot!  The  northern 
hemisphere  shows  a  regular  diminution  in  earthquake 
frequency  from  December  until  June,  and  an  increase 
the  rest  of  the  year.  In  the  southern  hemisphere  the 
course  of  events  is  the  same  so  far  as  summer  and  winter 
are  concerned,  for  August  with  its  maximum  comes  in 
winter,  while  February  with  its  minimum  comes  in 
summer.  In  the  southern  hemisphere  the  winter  month 
of  greatest  seismic  activity  has  over  100  per  cent  more 
earthquakes  than  the  summer  month  of  least  activity.  In 
the  northern  hemisphere  this  difference  is  about  80  per 
cent,  but  this  smaller  figure  occurs  partly  because  the 
northern  data  include  certain  interesting  and  signifi- 
cant regions  like  Japan  and  China  where  the  usual  condi- 
tions are  reversed.7  If  equatorial  regions  were  included 
in  Fig.  12,  they  would  give  an  almost  straight  line. 

The  connection  between  earthquakes  and  the  seasons  is 
so  strong  that  almost  no  students  of  seismology  question 
it,  although  they  do  not  agree  as  to  its  cause.  A  meteoro- 
logical hypothesis  seems  to  be  the  only  logical  explana- 
tion.8 Wherever  sufficient  data  are  available,  earthquakes 

iN.  F.  Drake:  Destructive  Earthquakes  in  China;  Bull.  Seism.  Soc.  Am., 
Vol.  2,  1912,  pp.  40-91,  124-133. 

8  The  only  other  explanation  that  seems  to  have  any  standing  is  the 
psychological  hypothesis  of  Montessus  de  Ballore  as  given  in  Les  Tremble- 
ments  de  Terre.  He  attributes  the  apparent  seasonal  variation  in  earth- 
quakes to  the  fact  that  in  winter  people  are  within  doors,  and  hence  notice 


298  CLIMATIC  CHANGES 

appear  to  be  most  numerous  when  climatic  conditions 
cause  the  earth 's  surface  to  be  most  heavily  loaded  or  to 
change  its  load  most  rapidly.  The  main  factor  in  the 
loading  is  apparently  atmospheric  pressure.  This  acts  in 
two  ways.  First,  when  the  continents  become  cold  in 
winter  the  pressure  increases.  On  an  average  the  air 
at  sea  level  presses  upon  the  earth's  surface  at  the  rate 
of  14.7  pounds  per  square  inch,  or  over  a  ton  per  square 
foot,  and  only  a  little  short  of  thirty  million  tons  per 
square  mile.  An  average  difference  of  one  inch  between 
the  atmospheric  pressure  of  summer  and  winter  over  ten 
million  square  miles  of  the  continent  of  Asia,  for  ex- 
ample, means  that  the  continent's  load  in  winter  is  about 
ten  million  million  tons  heavier  than  in  summer.  Second, 
the  changes  in  atmospheric  pressure  due  to  the  passage 
of  storms  are  relatively  sharp  and  sudden.  Hence  they 
are  probably  more  effective  than  the  variations  in  the 
load  from  season  to  season.  This  is  suggested  by  the 
rapidity  with  which  the  terrestrial  response  seems  to 
follow  the  supposed  solar  cause  of  earthquakes.  It  is  also 
suggested  by  the  fact  that  violent  storms  are  frequently 
followed  by  violent  earthquakes. ' '  Earthquake  weather, ' ' 
as  Dr.  Schlesinger  suggests,  is  a  common  phrase  in  the 
typhoon  region  of  Japan,  China,  and  the  East  Indies. 
During  tropical  hurricanes  a  change  of  pressure  amount- 
ing to  half  an  inch  in  two  hours  is  common.  On  Septem- 

"  movements  of  the  earth  much  more  than  in  summer  when  they  are  out  of 
doors.  There  is  a  similar  difference  between  people's  habits  in  high  lati- 
tudes and  low.  Undoubtedly  this  does  have  a  marked  effect  upon  the  degree 
to  which  minor  earthquake  shocks  are  noticed.  Nevertheless,  de  Ballore's 
contention,  as  well  as  any  other  psychological  explanation,  is  completely 
upset  by  two  facts:  First,  instrumental  records  show  the  same  seasonal  dis- 
tribution as  do  records  based  on  direct  observation,  and  instruments  cer- 
tainly are  not  influenced  by  the  seasons.  Second,  in  some  places,  notably 
China,  as  Drake  has  shown,  the  summer  rather  than  the  winter  is  very 
decidedly  the  time  when  earthquakes  are  most  frequent. 


THE  EARTH'S  CRUST  AND  THE  SUN 


299 


ber  22,  1885,  at  False  Point  Lighthouse  on  the  Bay  of 
Bengal,  the  barometer  fell  about  an  inch  in  six  hours, 
then  nearly  an  inch  and  a  half  in  not  much  over  two 
hours,  and  finally  rose  fully  two  inches  inside  of  two 
hours.  A  drop  of  two  inches  in  barometric  pressure 
means  that  a  load  of  about  two  million  tons  is  removed 


I  I 


i  I 


I  I 


Fig.  12.  Seasonal  distribution  of  earthquakes. 

(After  Davisson  and  Knott.) 
Northern  Hemisphere.         Southern  Hemisphere. 


300  CLIMATIC  CHANGES 

from  each  square  mile  of  land ;  the  corresponding  rise  of 
pressure  means  the  addition  of  a  similar  load.  Such  a 
storm,  and  to  a  less  degree  every  other  storm,  strikes  a 
blow  upon  the  earth 's  surface,  first  by  removing  millions 
of  tons  of  pressure  and  then  by  putting  them  on  again.9 
Such  storms,  as  we  have  seen,  are  much  more  frequent 
and  severe  when  sunspots  are  numerous  than  at  other 
times.  Moreover,  as  Veeder10  long  ago  showed,  one  of  the 
most  noteworthy  evidences  of  a  connection  between  sun- 
spots  and  the  weather  is  a  sudden  increase  of  pressure  in 
certain  widely  separated  high  pressure  areas.  In  most 
parts  of  the  world  winter  is  not  only  the  season  of 
highest  pressure  and  of  most  frequent  changes  of 
Veeder 's  type,  but  also  of  severest  storms.  Hence  a 
meteorological  hypothesis  would  lead  to  the  expectation 
that  earthquakes  would  occur  more  frequently  in  winter 
than  in  summer.  On  the  Chinese  coast,  however,  and  also 
on  the  oceanic  side  of  Japan,  as  well  as  in  some  more 
tropical  regions,  the  chief  storms  come  in  summer  in  the 
form  of  typhoons.  These  are  the  places  where  earth- 
quakes also  are  most  abundant  in  summer.  Thus,  wher- 
ever we  turn,  storms  and  the  related  barometric  changes 
seem  to  be  most  frequent  and  severe  at  the  very  times 
when  earthquakes  are  also  most  frequent. 

Other   meteorological    factors,    such    as    rain,    snow, 
winds,  and  currents,  probably  have  some  effect  on  earth- 

9  A  comparison  of  tropical  hurricanes  with  earthquakes  is  interesting. 
Taking  all  the  hurricanes  recorded  in  August,  September,  and  October,  from 
1880  to  1899,  and  the  corresponding  earthquakes  in  Milne's  catalogue,  the 
correlation  coefficient  between  hurricanes  and  earthquakes  is  -)-0.236,  with  a 
probable  error  of  ±0.082,  the  month  being  used  as  the  unit.  This  is  not  a 
large  correlation,  yet  when  it  is  remembered  that  the  hurricanes  represent 
only  a  small  part  of  the  atmospheric  disturbances  in  any  given  month,  it 
suggests  that  with  fuller  data  the  correlation  might  be  large. 

10  Ellsworth  Huntington:   The  Geographic  Work  of  Dr.  M.  A.  Veeder; 
Geog.  Rev.,  Vol.  3,  March  and  April,  1917,  Nos.  3  and  4. 


THE  EARTH'S  CRUST  AND  THE  SUN    301 

quakes  through  their  ability  to  load  the  earth's  crust. 
The  coming  of  vegetation  may  also  help.  These  agencies, 
however,  appear  to  be  of  small  importance  compared 
with  the  storms.  In  high  latitudes  and  in  regions  of 
abundant  storminess  most  of  these  factors  generally 
combine  with  barometric  pressure  to  produce  frequent 
changes  in  the  load  of  the  earth's  crust,  especially  in 
winter.  In  low  latitudes,  on  the  other  hand,  there  are  few 
severe  storms,  and  relatively  little  contrast  in  pressure 
and  vegetation  from  season  to  season ;  there  is  no  snow ; 
and  the  amount  of  ground  water  changes  little.  With  this 
goes  the  twofold  fact  that  there  is  no  marked  seasonal 
distribution  of  earthquakes,  and  that  except  in  certain 
local  volcanic  areas,  earthquakes  appear  to  be  rare.  In 
proportion  to  the  areas  concerned,  for  example,  there  is 
little  evidence  of  earthquakes  in  equatorial  Africa  and 
South  America. 

The  question  of  the  reality  of  the  connection  between 
meteorological  conditions  and  crustal  movements  is  so 
important  that  every  possible  test  should  be  applied.  At 
the  suggestion  of  Professor  Schlesinger  we  have  looked 
up  a  very  ingenious  line  of  inquiry.  During  the  last 
decades  of  the  nineteenth  century,  a  long  series  of  ex- 
tremely accurate  observations  of  latitude  disclosed  a  fact 
which  had  previously  been  suspected  but  not  demon- 
strated, namely,  that  the  earth  wabbles  a  little  about  its 
axis.  The  axis  itself  always  points  in  the  same  direction, 
and  since  the  earth  slides  irregularly  around  it  the  lati- 
tude of  all  parts  of  the  earth  keeps  changing.  Chandler 
has  shown  that  the  wabbling  thus  induced  consists  of 
two  parts.  The  first  is  a  movement  in  a  circle  with  a 
radius  of  about  fifteen  feet  which  is  described  in  approxi- 
mately 430  days.  This  so-called  Eulerian  movement  is  a 
normal  gyroscopic  motion  like  the  slow  gyration  of  a 


302  CLIMATIC  CHANGES 

spinning  top.  This  depends  on  purely  astronomical 
causes,  and  no  terrestrial  cause  can  stop  it  or  eliminate 
it.  The  period  appears  to  be  constant,  but  there  are  cer- 
tain puzzling  irregularities.  The  usual  amplitude  of  this 
movement,  as  Schlesinger11  puts  it,  "is  about  0".27,  but 
twice  in  recent  years  it  has  jumped  to  0".40.  Such  a 
change  could  be  accounted  for  by  supposing  that  the 
earth  had  received  a  severe  blow  or  a  series  of  milder 
blows  tending  in  the  same  direction. ' '  These  blows,  which 
were  originally  suggested  by  Helmert  are  most  interest- 
ing in  view  of  our  suggestion  as  to  the  blows  struck  by 
storms. 

The  second  movement  of  the  pole  has  a  period  of  a 
year,  and  is  roughly  an  ellipse  whose  longest  radius  is 
fourteen  feet  and  the  shortest,  four  feet;  or,  to  put  it 
technically,  there  is  an  annual  term  with  a  maximum 
amplitude  of  about  0".20.  This,  however,  varies  irregu- 
larly. The  result  is  that  the  pole  seems  to  wander  over 
the  earth's  surface  in  the  spiral  fashion  illustrated  in 
Fig.  13.  It  was  early  suggested  that  this  peculiar  wan- 
dering of  the  pole  in  an  annual  period  must  be  due  to 
meteorological  causes.  Jeffreys12  has  investigated  the 
matter  exhaustively.  He  assumes  certain  reasonable 
values  for  the  weight  of  air  added  or  subtracted  from 
different  parts  of  the  earth's  surface  according  to  the 
seasons.  He  also  considers  the  effect  of  precipitation, 
vegetation,  and  polar  ice,  and  of  variations  of  tempera- 
ture and  atmospheric  pressure  in  their  relation  to  move- 
ments of  the  ocean.  Then  he  proceeds  to  compare  all 

"Frank  Schlesinger:  Variations  of  Latitude;  Their  Bearing  upon  Our 
Knowledge  of  the  Interior  of  the  Earth;  Proc.  Am.  Phil.  Soc.,  Vol.  54, 
1915,  pp.  351-358.  Also  Smithsonian  Eeport  for  1916,  pp.  248-254. 

12  Harold  Jeffreys :  Causes  Contributory  to  the  Annual  Variations  of 
Latitude;  Monthly  Notices,  Eoyal  Astronomical  Soc.,  Vol.  76,  1916,  pp. 
499-525. 


THE  EARTH'S  CRUST  AND  THE  SUN 


303 


these  with  the  actual  wandering  of  the  pole  from  1907 
to  1913.  While  it  is  as  yet  too  early  to  say  that  any 
special  movement  of  the  pole  was  due  to  the  specific 
meteorological  conditions  of  any  particular  year, 
Jeffreys '  work  makes  it  clear  that  meteorological  causes, 
especially  atmospheric  pressure,  are  sufficient  to  cause 
the  observed  irregular  wanderings.  Slight  wanderings 
may  arise  from  various  other  sources  such  as  movements 
of  the  rocks  when  geological  faults  occur  or  the  rush  of 
a  great  wave  due  to  a  submarine  earthquake.  So  far  as 


Fig.  13.  Wandering  of  the 
pole  from  1890  to  1898. 

(After  Moulton.) 

known,  however,  all  these  other  agencies  cause  insignifi- 
cant displacements  compared  with  those  arising  from 
movements  of  the  air.  This  fact  coupled  with  the  mathe- 
matical certainty  that  meteorological  phenomena  must 
produce  some  wandering  of  the  pole,  has  caused  most 
astronomers  to  accept  Jeffreys'  conclusion.  If  we  follow 
their  example  we  are  led  to  conclude  that  changes  in 
atmospheric  pressure  and  in  the  other  meteorological 
conditions  strike  blows  which  sometimes  shift  the  earth 


304  CLIMATIC  CHANGES 

several  feet  from  its  normal  position  in  respect  to  the 
axis. 

If  the  foregoing  reasoning  is  correct,  the  great  and 
especially  the  sudden  departures  from  the  smooth 
gyroscopic  circle  described  by  the  pole  in  the  Eulerian 
motion  would  be  expected  to  occur  at  about  the  same  time 
as  unusual  earthquake  activity.  This  brings  us  to  an 
interesting  inquiry  carried  out  by  Milne13  and  amplified 
by  Knott.14  Taking  Albrecht's  representation  of  the 
irregular  spiral-like  motion  of  the  pole,  as  given  in  Fig. 
13,  they  show  that  there  is  a  preponderance  of  severe 
earthquakes  at  times  when  the  direction  of  motion  of  the 
earth  in  reference  to  its  axis  departs  from  the  smooth 
Eulerian  curve.  A  summary  of  their  results  is  given  in 
Table  9.  The  table  indicates  that  during  the  period  from 
1892  to  1905  there  were  nine  different  times  when  the 
curve  of  Fig.  13  changed  its  direction  or  was  deflected  by 
less  than  10°  during  a  tenth  of  a  year.  In  other  words, 
during  those  periods  it  did  not  curve  as  much  as  it  ought 
according  to  the  Eulerian  movement.  At  such  times  there 
were  179  world-shaking  earthquakes,  or  an  average  of 
about  19.9  per  tenth  of  a  year.  According  to  the  other 
lines  of  Table  9,  in  thirty-two  cases  the  deflection  during 
a  tenth  of  a  year  was  between  10°  and  25°,  while  in  fifty- 
six  cases  it  was  from  25°  to  40°.  During  these  periods 
the  curve  remained  close  to  the  Eulerian  path  and  the 
world-shaking  earthquakes  averaged  only  8.2  and  12.9. 
Then,  when  the  deflection  was  high,  that  is,  when  meteoro- 
logical conditions  threw  the  earth  far  out  of  its  Eule- 
rian course,  the  earthquakes  were  again  numerous,  the 
number  rising  to  23.4  when  the  deflection  amounted  to 
more  than  55°. 

i3  John  Milne:  British  Association  Eeports  for  1903  and  1906. 

"  C.  G.  Knott:  The  Physics  of  Earthquake  Phenomena,  Oxford,  1908. 


THE  EARTH'S  CRUST  AND  THE  SUN    305 


TABLE  9 

DEFLECTION  OF  PATH  OF  POLE  COMPARED 

WITH  EARTHQUAKES 

No.  of               No.  of             Average  No. 

Deflection 

Deflections       Earthquakes      of  Earthquakes 

0-10° 

9                         179                         19.9 

10-25° 

32                         263                           8.2 

25-40° 

56                         722                         12.9 

40-55° 

19                         366                         19.3 

over  55° 

7                         164                         23.4 

In  order  to  test  this  conclusion  in  another  way  we  have 
followed  a  suggestion  of  Professor  Schlesinger.  Under 
his  advice  the  Eulerian  motion  has  been  eliminated  and 
a  new  series  of  earthquake  records  has  been  compared 
with  the  remaining  motions  of  the  poles  which  presum- 
ably arise  largely  from  meteorological  causes.  For  this 
purpose  use  has  been  made  of  the  very  full  records  of 
earthquakes  published  under  the  auspices  of  the  Inter- 
national Seismological  Commission  for  the  years  1903 
to  1908,  the  only  years  for  which  they  are  available. 
These  include  every  known  shock  of  every  description 
which  was  either  recorded  by  seismographs  or  by  direct 
observation  in  any  part  of  the  world.  Each  shock  is  given 
the  same  weight,  no  matter  what  its  violence  or  how 
closely  it  follows  another.  The  angle  of  deflection  has 
been  measured  as  Milne  measured  it,  but  since  the  Eule- 
rian motion  is  eliminated,  our  zero  is  approximately 
the  normal  condition  which  would  prevail  if  there  were 
no  meteorological  complications.  Dividing  the  deflections 
into  six  equal  groups  according  to  the  size  of  the  angle, 
we  get  the  result  shown  in  Table  10. 


306  CLIMATIC  CHANGES 


TABLE  10 

EARTHQUAKES  IN  1903-1908  COMPARED 
WITH  DEPARTURES  OF  THE  PRO- 
JECTED CURVE  OF  THE  EARTH'S 
AXIS   FROM  THE   EULE- 
RIAN  POSITION 


Average  angle  of  deflection          Average  dotty  number 
(10  periods  of  ^Q  year  each)  of  earthquakes 

—10.5°  /^/f      8.31 

11.5° 

25.8° 

40.2° 

54.7' 

90.; 


Here  where  some  twenty  thousand  earthquakes  are 
employed  the  result  agrees  closely  with  that  of  Milne  for 
a  different  series  of  years  and  for  a  much  smaller  number 
of  earthquakes.  So  long  as  the  path  of  the  pole  departs 
less  than  about  45°  from  the  smooth  gyroscopic  Eulerian 
path,  the  number  of  earthquakes  is  almost  constant,  about 
eight  and  a  quarter  per  day.  When  the  angle  becomes 
large,  however,  the  number  increases  by  nearly  50  per 
cent.  Thus  the  work  of  Milne,  Knott,  and  Jeffreys  is  con- 
firmed by  a  new  investigation.  Apparently  earthquakes 
and  crustal  movements  are  somehow  related  to  sudden 
changes  in  the  load  imposed  on  the  earth's  crust  by 
meteorological  conditions. 

This  conclusion  is  quite  as  surprising  to  the  authors 
as  to  the  reader — perhaps  more  so.  At  the  beginning  of 
this  investigation  we  had  no  faith  whatever  in  any  im- 


THE  EARTH'S  CRUST  AND  THE  SUN        307 

portant  relation  between  climate  and  earthquakes.  At  its 
end  we  are  inclined  to  believe  that  the  relation  is  close 
and  important. 

It  must  not  be  supposed,  however,  that  meteorological 
conditions  are  the  cause  of  earthquakes  and  of  move- 
ments of  the  earth's  crust.  Even  though  the  load  that  the 
climatic  agencies  can  impose  upon  the  earth's  crust  runs 
into  millions  of  tons  per  square  mile,  it  is  a  trifle  com- 
pared with  what  the  crust  is  able  to  support.  There  is, 
however,  a  great  difference  between  the  cause  and  the 
occasion  of  a  phenomenon.  Suppose  that  a  thick  sheet  of 
glass  is  placed  under  an  increasing  strain.  If  the  strain 
is  applied  slowly  enough,  even  so  rigid  a  material  as  glass 
will  ultimately  bend  rather  than  break.  But  suppose  that 
while  the  tension  is  high  the  glass  is  tapped.  A  gentle 
tap  may  be  followed  by  a  tiny  crack".  A  series  of  little 
taps  may  be  the  signal  for  small  cracks  to  spread  in 
every  direction.  A  few  slightly  harder  taps  may  cause 
the  whole  sheet  to  break  suddenly  into  many  pieces.  Yet 
even  the  hardest  tap  may  be  the  merest  trifle  compared 
with  the  strong  force  which  is  keeping  the  glass  in  a  state 
of  strain  and  which  would  ultimately  bend  it  if  given 
time. 

The  earth  as  a  whole  appears  to  stand  between  steel 
and  glass  in  rigidity.  It  is  a  matter  of  common  observa- 
tion that  rocks  stand  high  in  this  respect  and  in  the 
consequent  difficulty  with  which  they  can  be  bent  without 
breaking.  Because  of  the  earth's  contraction  the  crust 
endures  a  constant  strain,  which  must  gradually  become 
enormous.  This  strain  is  increased  by  the  fact  that  sedi- 
ment is  transferred  from  the  lands  to  the  borders  of  the 
sea  and  there  forms  areas  of  thick  accumulation.  From 
this  has  arisen  the  doctrine  of  isostasy,  or  of  the  equali- 
zation of  crustal  pressure.  An  important  illustration  of 


308  CLIMATIC  CHANGES 

this  is  the  oceanward  and  equatorial  creep  which  has 
been  described  in  Chapter  XI.  There  we  saw  that  when 
the  lands  have  once  been  raised  to  high  levels  or  when  a 
shortening  of  the  earth's  axis  by  contraction  has  in- 
creased the  oceanic  bulge  at  the  equator,  or  when  the 
reverse  has  happened  because  of  tidal  retardation,  the 
outer  part  of  the  earth  appears  to  creep  slowly  back 
toward  a  position  of  perfect  isostatic  adjustment.  If  the 
sun  had  no  influence  upon  the  earth,  either  direct  or 
indirect,  isostasy  and  other  terrestrial  processes  might 
flex  the  earth's  crust  so  gradually  that  changes  in  the 
form  and  height  of  the  lands  would  always  take  place 
slowly,  even  from  the  geological  point  of  view.  Thus 
erosion  would  usually  be  able  to  remove  the  rocks  as 
rapidly  as  they  were  domed  above  the  general  level.  If 
this  happened,  mountains  would  be  rare  or  unknown,  and 
hence  climatic  contrasts  would  be  far  less  marked  than  is 
actually  the  case  on  our  earth  where  crustal  movements 
have  repeatedly  been  rapid  enough  to  produce  mountains. 
Nature's  methods  rarely  allow  so  gradual  an  adjust- 
ment to  the  forces  of  isostasy.  While  the  crust  is  under  a 
strain,  not  only  because  of  contraction,  but  because  of 
changes  in  its  load  through  the  transference  of  sediments 
and  the  slow  increase  or  decrease  in  the  bulge  at  the 
equator,  the  atmosphere  more  or  less  persistently  carries 
on  the  tapping  process.  The  violence  of  that  process 
varies  greatly,  and  the  variations  depend  largely  on  the 
severity  of  the  climatic  contrasts.  If  the  main  outlines  of 
the  cyclonic  hypothesis  are  reliable,  one  of  the  first  effects 
of  a  disturbance  of  the  sun's  atmosphere  is  increased 
storminess  upon  the  earth.  This  is  accompanied  by  in- 
creased intensity  in  almost  every  meteorological  process. 
The  most  important  effect,  however,  so  far  as  the  earth's 
crust  is  concerned  would  apparently  be  the  rapid  and 


THE  EARTH'S  CRUST  AND  THE  SUN    309 

intense  changes  of  atmospheric  pressure  which  would 
arise  from  the  swift  passage  of  one  severe  storm  after 
another.  Each  storm  would  be  a  little  tap  on  the  tensely 
strained  crust.  Any  single  tap  might  be  of  little  conse- 
quence, even  though  it  involved  a  change  of  a  billion 
tons  in  the  pressure  on  an  area  no  larger  than  the  state 
of  Rhode  Island.  Yet  a  rapid  and  irregular  succession  of 
such  taps  might  possibly  cause  the  crust  to  crack,  and 
finally  to  collapse  in  response  to  stresses  arising  from 
the  shrinkage  of  the  earth. 

Another  and  perhaps  more  important  effect  of  varia- 
tions in  storminess  and  especially  in  the  location  of  the 
stormy  areas  would  be  an  acceleration  of  erosion  in  some 
places  and  a  retardation  elsewhere.  A  great  increase  in 
rainfall  may  almost  denude  the  slopes  of  soil,  while  a 
diminution  to  the  point  where  much  of  the  vegetation 
dies  off  has  a  similar  effect.  If  such  changes  should  take 
place  rapidly,  great  thicknesses  of  sediment  might  be 
concentrated  in  certain  areas  in  a  short  time,  thus  dis- 
turbing the  isostatic  adjustment  of  the  earth's  crust.  This 
might  set  up  a  state  of  strain  which  would  ultimately 
have  to  be  relieved,  thus  perhaps  initiating  profound 
crustal  movements.  Changes  in  the  load  of  the  earth's 
crust  due  to  erosion  and  the  deposition  of  sediment,  no 
matter  how  rapid  they  may  be  from  the  geological  stand- 
point, are  slow  compared  with  those  due  to  changes  in 
barometric  pressure.  A  drop  of  an  inch  in  barometric 
pressure  is  equivalent  to  the  removal  of  about  five  inches 
of  solid  rock.  Even  under  the  most  favorable  circum- 
stances, the  removal  of  an  average  depth  of  five  inches 
of  rock  or  its  equivalent  in  soil  over  millions  of  square 
miles  would  probably  take  several  hundred  years,  while 
the  removal  of  a  similar  load  of  air  might  occur  in  half 
a  day  or  even  a  few  hours.  Thus  the  erosion  and  depo- 


310  CLIMATIC  CHANGES 

sition  due  to  climatic  variations  presumably  play  their 
part  in  crustal  deformation  chiefly  by  producing  crustal 
stresses,  while  the  storms,  as  it  were,  strike  sharp,  sudden 
blows. 

Suppose  now  that  a  prolonged  period  of  world-wide 
mild  climate,  such  as  is  described  in  Chapter  X,  should 
permit  an  enormous  accumulation  of  stresses  due  to  con- 
traction and  tidal  retardation.  Suppose  that  then  a 
sudden  change  of  climate  should  produce  a  rapid  shifting 
of  the  deep  soil  that  had  accumulated  on  the  lands,  with  a 
corresponding  localization  and  increase  in  strains.  Sup- 
pose also  that  frequent  and  severe  storms  play  their  part, 
whether  great  or  small,  by  producing  an  intensive  tapping 
of  the  crust.  In  such  a  case  the  ultimate  collapse  would 
be  correspondingly  great,  as  would  be  evident  in  the 
succeeding  geological  epoch.  The  sea  floor  might  sink 
lower,  the  continents  might  be  elevated,  and  mountain 
ranges  might  be  shoved  up  along  lines  of  special  weak- 
ness. This  is  the  story  of  the  geological  period  as  known 
to  historical  geology.  The  force  that  causes  such  move- 
ments would  be  the  pull  of  gravity  upon  the  crust  sur- 
rounding the  earth's  shrinking  interior.  Nevertheless 
climatic  changes  might  occasionally  set  the  date  when  the 
gravitative  pull  would  finally  overcome  inertia,  and  thus 
usher  in  the  crustal  movements  that  close  old  geologic 
periods  and  inaugurate  new  ones.  This,  however,  could 
occur  only  if  the  crust  were  under  sufficient  strain.  As 
Lawson15  says  in  his  discussion  of  the  "elastic  rebound 
theory,"  the  sudden  shifts  of  the  crust  which  seem  to  be 
the  underlying  cause  of  earthquakes  "can  occur  only 
after  the  accumulation  of  strain  to  a  limit  and  .  .  .  this 
accumulation  involves  a  slow  creep  of  the  region  affected. 

IB  A.  C.  Lawson:  The  Mobility  of  the  Coast  Eanges  of  California;  Univ. 
of  Calif.  Pub.,  Geology,  Vol.  12,  No.  7,  pp.  431-4-73. 


THE  EARTH'S  CRUST  AND  THE  SUN    311 

In  the  long  periods  between  great  earthquakes  the  energy 
necessary  for  such  shocks  is  being  stored  up  in  the  rocks 
as  elastic  compression." 

If  a  period  of  intense  storminess  should  occur  when 
the  earth  as  a  whole  was  in  such  a  state  of  strain,  the 
sudden  release  of  the  strains  might  lead  to  terrestrial 
changes  which  would  alter  the  climate  still  further,  mak- 
ing it  more  extreme,  and  perhaps  permitting  the  stormi- 
ness due  to  the  solar  disturbances  to  bring  about  gla- 
ciation.  At  the  same  time  if  volcanic  activity  should 
increase  it  would  add  its  quota  to  the  tendency  toward 
glaciation.  Nevertheless,  it  might  easily  happen  that  a 
very  considerable  amount  of  crustal  movement  would 
take  place  without  causing  a  continental  ice  sheet  or  even 
a  marked  alpine  ice  sheet.  Or  again,  if  the  strains  in  the 
earth's  crust  had  already  been  largely  released  through 
other  agencies  before  the  stormy  period  began,  the  cli- 
mate might  become  severe  enough  to  cause  glaciation 
in  high  latitudes  without  leading  to  any  very  marked 
movements  of  the  earth's  crust,  as  apparently  happened 
in  the  Mid- Silurian  period. 


CONCLUSION 

Here  we  must  bring  this  study  of  the  earth's  evolution 
to  a  close.  Its  fundamental  principle  has  been  that  the 
present,  if  rightly  understood,  affords  a  full  key  to  the 
past.  With  this  as  a  guide  we  have  touched  on  many 
hypotheses,  some  essential  and  some  unessential  to  the 
general  line  of  thought.  The  first  main  hypothesis  is  that 
the  earth's  present  climatic  variations  are  correlated 
with  changes  in  the  solar  atmosphere.  This  is  the  key- 
note of  the  whole  book.  It  is  so  well  established,  however, 


312  CLIMATIC  CHANGES 

that  it  ranks  as  a  theory  rather  than  as  an  hypothesis. 
Next  comes  the  hypothesis  that  variations  in  the  solar 
atmosphere  influence  the  earth's  climate  chiefly  by  caus- 
ing variations  not  only  in  temperature  but  also  in 
atmospheric  pressure  and  thus  in  storminess,  wind,  and 
rainfall.  This,  too,  is  one  of  the  essential  foundations  on 
which  the  rest  of  the  book  is  built,  but  though  this 
cyclonic  hypothesis  is  still  a  matter  of  discussion,  it 
seems  to  be  based  on  strong  evidence.  These  two  hypothe- 
ses might  lead  us  astray  were  they  not  balanced  by 
another.  This  other  is  that  many  climatic  conditions  are 
due  to  purely  terrestrial  causes,  such  as  the  form  and 
altitude  of  the  lands,  the  degree  to  which  the  continents 
are  united,  the  movement  of  ocean  currents,  the  activity 
of  volcanoes,  and  the  composition  of  the  atmosphere  and 
the  ocean.  Only  by  combining  the  solar  and  the  terrestrial 
can  the  truth  be  perceived.  Finally,  the  last  main  hypothe- 
sis of  this  book  holds  that  if  the  climatic  conditions  which 
now  prevail  at  times  of  solar  activity  were  magnified 
sufficiently  and  if  they  occurred  in  conjunction  with  cer- 
tain important  terrestrial  conditions  of  which  there  is 
good  evidence,  they  would  produce  most  of  the  notable 
phenomena  of  glacial  periods.  For  example,  they  would 
explain  such  puzzling  conditions  as  the  localization  and 
periodicity  of  glaciation,  the  formation  of  loess,  and  the 
occurrence  of  glaciation  in  low  latitudes  during  Permian 
and  Proterozoic  times.  The  converse  of  this  is  that  if 
the  conditions  which  now  prevail  at  times  when  the  sun 
is  relatively  inactive  should  be  intensified,  that  is,  if 
the  sun's  atmosphere  should  become  calmer  than  now, 
and  if  the  proper  terrestrial  conditions  of  topographic 
form  and  atmospheric  composition  should  prevail,  there 
would  arise  the  mild  climatic  conditions  which  appear 
to  have  prevailed  during  the  greater  part  of  geological 


THE  EARTH'S  CRUST  AND  THE  SUN    313 

time.  In  short,  there  seems  thus  far  to  be  no  phase  of 
the  climate  of  the  past  which  is  not  in  harmony  with  an 
hypothesis  which  combines  into  a  single  unit  the  three 
main  hypotheses  of  this  book,  solar,  cyclonic,  and  terres- 
trial. 

Outside  the  main  line  of  thought  lie  several  other 
hypotheses.  Several  of  these,  as  well  as  some  of  the  main 
hypotheses,  are  discussed  chiefly  in  Earth  and  Sun,  but 
as  they  are  given  a  practical  application  in  this  book 
they  deserve  a  place  in  this  final  summary.  Each  of  these 
secondary  hypotheses  is  in  its  way  important.  Yet  any  or 
all  may  prove  untrue  without  altering  our  main  conclu- 
sions. This  point  cannot  be  too  strongly  emphasized,  for 
there  is  always  danger  that  differences  of  opinion  as  to 
minor  hypotheses  and  even  as  to  details  may  divert  at- 
tention from  the  main  point.  Among  the  non-essential 
hypotheses  is  the  idea  that  the  sun's  atmosphere  influ- 
ences that  of  the  earth  electrically  as  well  as  thermally. 
This  idea  is  still  so  new  that  it  has  only  just  entered  the 
stage  of  active  discussion,  and  naturally  the  weight  of 
opinion  is  against  it.  Although  not  necessary  to  the  main 
purpose  of  this  book,  it  plays  a  minor  role  in  the  chapter 
dealing  with  the  relation  of  the  sun  to  other  astronomical 
bodies.  It  also  has  a  vital  bearing  on  the  further  advance 
of  the  science  of  meteorology  and  the  art  of  weather 
forecasting.  Another  secondary  hypothesis  holds  that 
sunspots  are  set  in  motion  by  the  planets.  Whether  the 
effect  is  gravitational  or  more  probably  electrical,  or 
perhaps  of  some  other  sort,  does  not  concern  us  at  pres- 
ent, although  the  weight  of  evidence  seems  to  point 
toward  electronic  emissions.  This  question,  like  that  of 
the  relative  parts  played  by  heat  and  electricity  in  terres- 
trial climatic  changes,  can  be  set  aside  for  the  moment. 
What  does  concern  us  is  a  third  hypothesis,  namely,  that 


314  CLIMATIC  CHANGES 

afjthe  planets  really  determine  the  periodicity  of  sun- 
spots,  even  though  not  supplying  the  energy,  the  sun  in 
its  flight  through  space  must  have  been  repeatedly  and 
more  strongly  influenced  in  the  same  way  by  many  other 
heavenly  bodies.  In  that  case,  climatic  changes  like  those 
of  the  present,  but  sometimes  greatly  magnified,  have  pre- 
sumably arisen  because  of  the  constantly  changing  posi- 
tion of  the  solar  system  in  respect  to  other  parts  of  the 
universe.  Finally,  the  fourth  of  our  secondary  hypotheses 
postulates  that  at  present  the  date  of  movements  of  the 
earth's  crust  is  often  determined  by  the  fact  that  storms 
and  other  meteorological  conditions  keep  changing  the 
load  upon  first  one  part  of  the  earth's  surface  and  then 
upon  another.  Thus  stresses  that  have  accumulated  in  the 
earth's  isostatic  shell  during  the  preceding  months  are 
released.  In  somewhat  the  same  way  epochs  of  extreme 
storminess  and  rapid  erosion  in  the  past  may  possibly 
have  set  the  date  for  great  movements  of  the  earth's 
crust.  This  hypothesis,  like  the  other  three  in  our  secon- 
dary or  non-essential  group,  is  still  so  new  that  only  the 
first  steps  have  been  taken  in  testing  it.  Yet  it  seems  to 
deserve  careful  study. 

In  testing  all  the  hypotheses  here  discussed,  primary 
and  secondary  alike,  the  first  necessity  is  a  far  greater 
amount  of  quantitative  work.  In  this  book  there  has  been 
a  constant  attempt  to  subject  every  hypothesis  to  the  test 
of  statistical  facts  of  observation.  Nevertheless,  we  have 
been  breaking  so  much  new  ground  that  in  many  cases 
exact  facts  are  not  yet  available,  while  in  others  they 
can  be  properly  investigated  only  by  specialists  in 
physics,  astronomy,  or  mathematics.  In  most  cases  the 
next  great  step  is  to  ascertain  whether  the  forces  here 
called  upon  are  actually  great  enough  to  produce  the 
observed  results.  Even  though  they  act  only  as  a  means 


THE  EARTH'S  CRUST  AND  THE  SUN    315 

of  releasing  the  far  greater  forces  due  to  the  contraction 
of  the  earth  and  the  sun,  .they  need  to  be  rigidly  tested 
as  to  their  ability  to  play  even  this  minor  role.  Still 
another  line  of  study  that  cries  aloud  for  research  is  a 
fuller  comparison  between  earthquakes  on  the  one  hand 
and  meteorological  conditions  and  the  wandering  of  the 
poles  on  the  other.  Finally,  an  extremely  interesting  and 
hopeful  quest  is  the  determination  of  the  positions  and 
movements  of  additional  stars  and  other  celestial  bodies, 
the  faint  and  invisible  as  well  as  the  bright,  in  order  to 
ascertain  the  probable  magnitude  of  their  influence  upon 
the  sun  and  thus  upon  the  earth  at  various  times  in  the 
past  and  in  the  future.  Perhaps  we  are  even  now  ap- 
proaching some  star  that  will  some  day  give  rise  to  a 
period  of  climatic  stress  like  that  of  the  fourteenth  cen- 
tury, or  possibly  to  a  glacial  epoch.  Or  perhaps  the  varia- 
tions in  others  of  the  nearer  stars  as  well  as  Alpha 
Centauri  may  show  a  close  relation  to  changes  in  the  sun. 
Throughout  this  volume  we  have  endeavored  to  dis- 
cover new  truth  concerning  the  physical  environment 
that  has  molded  the  evolution  of  all  life.  We  have  seen 
how  delicate  is  the  balance  among  the  forces  of  nature, 
even  though  they  be  of  the  most  stupendous  magnitude. 
We  have  seen  that  a  disturbance  of  this  balance  in  one 
of  the  heavenly  bodies  may  lead  to  profound  changes  in 
another  far  away.  Yet  during  the  billion  years,  more  or 
less,  of  which  we  have  knowledge,  there  appears  never 
to  have  been  a  complete  cataclysm  involving  the  destruc- 
tion of  all  life.  One  star  after  another,  if  our  hypothesis 
is  correct,  has  approached  the  solar  system  closely 
enough  to  set  the  atmosphere  of  the  sun  in  such  commo- 
tion that  great  changes  of  climate  have  occurred  upon 
the  earth.  Yet  never  has  the  solar  system  passed  so  close 
to  any  other  body  or  changed  in  any  other  way  suffi- 


316  CLIMATIC  CHANGES 

ciently  to  blot  out  all  living  things.  The  effect  of  climatic 
changes  has  always  been  to  alter  the  environment  and 
therefore  to  destroy  part  of  the  life  of  a  given  time,  but 
with  this  there  has  invariably  gone  a  stimulus  to  other 
organic  types.  New  adaptations  have  occurred,  new  lines 
of  evolutionary  progress  have  been  initiated,  and  the  net 
result  has  been  greater  organic  diversity  and  richness. 
Temporarily  a  great  change  of  climate  may  seem  to 
retard  evolution,  but  only  for  a  moment  as  the  geologist 
counts  time.  Then  it  becomes  evident  that  the  march  of 
progress  has  actually  been  more  rapid  than  usual.  Thus 
the  main  periods  of  climatic  stress  are  the  most  conspicu- 
ous milestones  upon  the  upward  path  toward  more  varied 
adaptation.  The  end  of  each  such  period  of  stress  has 
found  the  life  of  the  world  nearer  to  the  high  mentality 
which  reaches  out  to  the  utmost  limits  of  space,  of  time, 
and  of  thought  in  the  search  for  some  explanation  of  the 
meaning  of  the  universe.  Each  approach  of  the  sun  to 
other  bodies,  if  such  be  the  cause  of  the  major  climatic 
changes,  has  brought  the  organic  world  one  step  nearer 
to  the  solution  of  the  greatest  of  all  problems, — the  prob- 
lem of  whether  there  is  a  psychic  goal  beyond  the  mental 
goal  toward  which  we  are  moving  with  ever  accelerating 
speed.  Throughout  the  vast  eons  of  geological  time  the 
adjustment  of  force  to  force,  of  one  body  of  matter  to 
another,  and  of  the  physical  environment  to  the  organic 
response  has  been  so  delicate,  and  has  tended  so  steadily 
toward  the  one  main  line  of  mental  progress  that  there 
seems  to  be  a  purpose  in  it  all.  If  the  cosmic  uniformity 
of  climate  continues  to  prevail  and  if  the  uniformity  is 
varied  by  changes  as  stimulating  as  those  of  the  past,  the 
imagination  can  scarcely  picture  the  wonders  of  the 
future.  In  the  course  of  millions  or  even  billions  of  years 
the  development  of  mind,  and  perhaps  of  soul,  many  excel 


THE  EARTH'S  CRUST  AND  THE  SUN        317 

that  of  today  as  far  as  the  highest  known  type  of  men- 
tality excels  the  primitive  plasma  from  which  all  life 
appears  to  have  arisen. 


INDEX 


Indicates   illustrations. 


Abbot,  0.  G.,  cited,  45,  52,  237,  238, 
239. 

Aboskun,  104. 

Africa,  earthquakes,  301;  East,  see 
East  Africa;  lakes,  143;  North, 
see  North  Africa. 

African  glaciation,  266. 

Air,  see  Atmosphere. 

Alaska,  glacial  till  in,  287;  Ice  Age 
in,  221. 

Albrecht,  cited,  304. 

Alexander,  march  of,  88  f . 

Allard,  H.  A.,  cited,  183,  184. 

Alpha  Centauri,  companion  of,  280; 
distance  from  sun,  262;  lumi- 
nosity, 278;  speed  of,  281;  varia- 
tions, 282. 

Alps,  loess  in,  159;  precipitation  in, 
141;  snow  level  in,  139. 

Altair,  companion  of,  280;  lumi- 
nosity, 278;  speed  of,  281. 

Amazon  forest,  temperature,  17. 

Ancylus  lake,  217. 

Andes,  snow  line,  139. 

Animals,  climate  and,  1. 

Antarctica,  mild  climate,  219;  thick- 
ness of  ice  in,  125;  winds,  135, 
161. 

Anti-cyclonic  hypothesis,  135  ff . 

Appalachians,  effect  on  ice  sheet, 
121. 

Arabia,  civilization  in,  67. 

Aral,  Sea  of,  108. 

Archean  rocks,  211. 

Archeozoic,  3  f.;  climate  of,  267. 

Arctic  Ocean,  submergence,  219. 

Arctowski,  H.,  cited,  29,  46,  244. 

Argon,  increase  of,  236. 


Arizona,  rainfall,  89,  108;  trees 
measured  in,  73. 

Arrhenius,  S.,  cited,  36,  254. 

Arsis,  of  pulsation,  24. 

Asbjorn  Selsbane,  corn  of,  101. 

Asia,  atmospheric  pressure,  298 ;  cen- 
tral, changes  of  climate,  *  75 ;  cen- 
tral, post-glacial  climate,  271; 
climate,  66;  glaciation  in,  131; 
storminess  in,  60;  western,  cli- 
mate in,  84  f. 

Atlantic  Ocean,  storminess,  57. 

Atmosphere,  changes,  19  f .,  229 ; 
composition  of,  223-241;  effect 
on  temperature,  231. 

Atmospheric  circulation,  glaciation 
and,  42. 

Atmospheric  electricity,  solar  rela- 
tions of,  56. 

Atmospheric  pressure,  earthquakes 
and,  298;  evaporation  and,  237; 
increase  in,  239;  redistribution  of, 
49;  variation,  53. 

Australia,  East,  mild  climate,  219; 
precipitation,  144. 

Axis,  earth's,  48;  wabbling  of,  301. 

Bacon,  Sir  Francis,  cited,  27. 

Bacubirito,  meteor  at,  246. 

Baltic   Sea,  as   lake,    217;    freezing 

of,  100;  ice,  26;  storm-floods,  99; 

submergence,  219. 
Bardsson,  Ivar,  106. 
Barkow,  cited,  135. 
Barometric  pressure,  solar  relations 

of,  56. 

Barrell,  J.,  cited,  3,  200,  213,  234. 
Bartoli,  A.  G.,  cited,  257. 


320 


INDEX 


Bauer,  L.  A.,  cited,  150. 
Beaches,  under  water,  97. 
Beadnell,  H.  J.  L.,  cited,  143. 
Beluchistan,  rainfall,  89. 
Bengal,  Bay  of,  cyclones  in,  149. 
Bengal,  famine  in,  104  f. 
Berlin,  rainfall  and  temperature,  93. 
Betelgeuse,     259  f . ;     distance    from 

sun,  262. 
Bible,    climatic    evidence    in,    91  f . ; 

palms  in,  92. 
Binary  stars,  252. 
Birkeland,  K.,  cited,  244. 
Black  Earth  region,  loess  in,  159. 
Boca,    Cal.,    correlation    coefficients, 

83,  85. 

Boltzmann,  L.,  cited,  257. 
Bonneville,  Lake,  142,  143. 
Borkum,  storm-flood  in,  99. 
Boss,  L.  cited,  268,  269. 
Botanical  evidence  of  mild  climates, 

167  ff. 

Boulders,  on  Irish  coast,  119. 
Bowie,  W.,  cited,  293. 
Bowman,  I.,  cited,  213. 
Britain,  forests,  220;  level  of  land, 

220. 
British   Isles,   height   of  land,    111; 

temperature,  216. 
Brooks,  C.  E.  P.,  cited,  115,  143,  196, 

215,  225. 

Brooks,  C.  F.,  cited,  209. 
Brown,  E.  W.,  cited,  191,  244. 
Bruckner,  E.,  cited,  27. 
Bruckner  periods,  27  f. 
Bufo,  habitat  of,  202. 
Buhl  stage,  216. 
Bull,  Dr.,  cited,  100,  101. 
Butler,   H.   C.,  cited,   66,  67  ff.,   70, 

76. 

California,  changes  of  climate,  *  75 ; 
correlations  of  rainfall,  86;  meas- 
urements of  sequoias  in,  73,  74  ff . ; 
rainfall,  108. 

Cambrian  period,  4  f . 

Canada,  storminess,  53  f .,  57 ;  storm 
tracks  in,  113. 

Cape  Farewell,  shore  ice  at,  105. 


Carbon  dioxide,  erosion  and,  119f.; 
from  volcanoes,  23;  hypothesis, 
139;  importance  of,  9,  111;  in 
Permian,  148;  in  atmosphere,  20, 
96,  238;  in  ocean,  226;  nebular 
hypothesis  and,  232;  theory  of 
glaciation,  36  ff. 

Caribbean  mountains,  origin  of,  193. 

Carnegie  Institution  of  Washington, 
74. 

Caspian  Sea,  climatic  stress,  104; 
rainfall,  107  f.;  rise  and  fall,  27; 
ruins  in,  71. 

Cenozoic,  climate,  266;  fossils,  21. 

Central  America,  Maya  ruins,  95. 

Chad,  Lake,  swamps  of,  171. 

Chamberlin,  E.  T.,  cited,  166,  233, 
269. 

Chamberlin,  T.  C.,  cited,  19,  36,  38, 
39,  42  f.,  48,  122,  125,  152,  156, 
190,  195,  227,  269. 

Chandler,  S.  C.,  cited,  301. 

Chinese  earthquakes,  periodicity  of, 
245. 

Chinese,  sunspot  observations,  108  f. 

Chinese  Turkestan,  desiccation  in, 
66. 

Chronology,  glacial,  215. 

Clarke,  F.  W.,  cited,  226,  235. 

Clayton,  H.  H.,  cited,  173  f. 

Climate,  effect  of  contraction, 
189  ff.;  effect  of  salinity,  224;  in 
history,  64-97;  uniformity,  1-15; 
variability,  16-32. 

Climates,  mild,  causes  of,  166-187; 
mild,  periods  of,  274. 

Climatic  changes,  and  crustal  move- 
ments, 285  ff.;  hypotheses  of,  33- 
50 ;  mountain-building  and,  *  25 ; 
post-glacial  crustal  movements 
and,  215-222;  terrestrial  causes 
of,  188-214. 

Climatic  sequence,  16  f . 

Climatic  stages,  post-glacial,  270. 

Climatic  stress,  in  fourteenth  cen- 
tury, 98-109. 

Climatic  uniformity,  hypothesis  of, 
65,  71  f . 

Climatic  zoning,  169. 


INDEX 


321 


Cloudiness,  glaciation  and,  114,  147. 

Clouds,  as  protection,  197. 

Colfax,  Cal.,  correlation  coefficients, 

83. 

Cologne,  flood  at,  99. 
Compass,  variations,  150. 
Continental  climate,  variations,  103. 
Continents,  effect  on  climate,  111  f. 
Contraction,  effect  on  climate,  189  ff., 

199,    207;    effect  on   lands,    207; 

heat  of  sun  and,  13  f . ;  irregular, 

195;  of  the  earth,  18;  of  the  sun, 

249;  stresses  caused  by,  310. 
Convection,  carbon  dioxide  and,  239. 
Corals,  in  high  latitudes,  21,  39,  167, 

178. 
Cordeiro,  F.  J.  B.,  cited,  181,  183, 

186. 
Correlation   coefficients,   earthquakes 

and  sunspots,  291;  Jerusalem  rain- 
fall   and    sequoia    growth,    83  ff. ; 

rainfall  and  tree  growth,  79  ff. 
Cosmos,  effect  of  light,  185. 
Cressey,  G.  B.,  cited,  80. 
Cretaceous,     lava,     211;     mountain 

ranges,  44;  paleogeography,  *  201; 

submergence    of    North    America, 

200. 

Croll,  J.,  cited,  34  ff.,  176. 
Croll's  hypothesis,  snow  line,  139. 
Crust,    climate    and    movements    of, 

63,  287,  310;  movements  of,  43; 

strains  in,  22. 

Currents  and  planetary  winds,  174. 
Cycads,  169. 
Cyclonic  hypothesis,   97;   loess  and, 

163;  Permian  glaciation  and,  148; 

snow  line,  139. 
Cyclonic   storms,    in   glacial   epochs, 

140  f.;    solar   electricity  and,  243 

(see  Storms,  Storminess). 
Cyclonic   vacillations,    30  f.;    nature 

of,  57  ff. 

Daily  vibrations,  28  f . 
Danube,  frozen,  98. 
Darwin,  G.  H.,  cited,  191. 
Daun  stage,  217. 
Davis,  W.  M.,  cited,  271. 


Davisson,  C.,  cited,  294,  295,  299. 

Day,  C.  P.,  cited,  239. 

Day,  length  of,  18,  191. 

Dead  Sea,  palms  near,  92. 

Death  Valley,  142. 

De  Ballore,  M.,  cited,  297,  298. 

Deep-sea  circulation,  rapidity,  227; 

salinity   and,   176;    solar   activity 

and,  179. 

De  Geer,  S.,  cited,  215,  221. 
De  Lapparent,  A.,  cited,  200. 
Denmark,  fossils,  271. 
"Desert  pavements,"  161. 
Deserts,  abundant  flora  of,  171 ;  and 

pulsations  theory,  88  ff . ;  red  beds 

of,  170. 
Devonian,  climate,   266;   mountains, 

209. 

Dog,  climate  and,  1. 
Donegal  County,  Ireland,  220. 
Double    stars,    272,    280;    electrical 

effect  of,  261. 
Douglass,  A.  E.,  cited,  28,  73,  74  f ., 

84,  85,  107. 
Dragon  Town,   destruction   of,   104, 

108. 

Drake,  N.  F.,  cited,  297,  298. 
Droughts,     and     pulsations     theory, 

87  f.;  in  England,  102;  in  India, 

104  f. 
Drumkelin  Bog,   Ireland,  log  cabin 

in,  220. 
Dust,  at  high  levels,  240. 

Earth,  crust  of  and  the  sun,  285-317; 
internal  heat,  212;  nature  of  mild 
climate,  274 ;  position  of  axis,  181 ; 
rigidity  of,  307;  temperature 
gradient,  213;  temperature  of  sur- 
face, 8. 

Earthquakes,  and  seasons,  294,  297; 
and  sunspots,  288  f . ;  and  tropical 
hurricanes,  300;  and  wandering 
of  pole,  304  f.;  cause  of,  307; 
compared  with  departures  from 
Eulerian  position,  306;  seasonal 
distribution  of,  299;  seasonal 
march,  295. 

"Earthquake  weather,"  298. 


322 


INDEX 


East  Africa,  mild  climate,  219. 

East  Indies,  earthquakes  of,  296. 

Eberswalde,  tree  growth  at,  102  f . 

Ecliptic,  obliquity  of,  217. 

Electrical  currents,  in  solar  atmos- 
phere, 261. 

Electrical  emissions,  variation  of, 
275. 

Electrical  hypothesis,  150,  250  f., 
256  ff. 

Electrical  phenomena,  storminess 
and,  56. 

Electricity,  and  earthquakes,  292; 
solar,  243. 

Electro-magnetic  hypothesis,  244. 

Electrons,  solar,  56;  variation  of, 
256. 

Electro-stellar  hypothesis,  274. 

Elevation,  climatic  changes  and,  39. 

Engedi,  palms  in,  92. 

England,  climatic  stress,  101  f . ; 
storminess  and  rainfall,  107. 

Eocene,  climate,  266. 

Equinoxes,  precession  of,  96. 

Erosion,  storminess  and,  309. 

Eskimo,  in  Greenland,  106. 

Eulerian  movement,  301,  304. 

Euphrates,  67. 

Europe,  climatic  stress,  98  ff.,  102  f . ; 
climatic  table,  215;  glaciation  in, 
131;  ice  sheet,  121;  inundations 
of  rivers,  99;  post-glacial  climate, 
271;  rainfall,  107;  submergence, 
196,  200. 

Evaporation,  and  glaeiation,  112, 
114;  atmospheric  pressure  and, 
237;  from  plants,  179;  impor- 
tance, 129;  in  trade-wind  belt, 
117;  rapidity  of,  224. 

Evening  primrose,  effect  of  light, 
184. 

Evolution,  climate  and,  20;  geo- 
graphical complexity  and,  241; 
glaciation  and,  33;  of  the  earth, 
311. 

Faculse,  cause  of,  61. 
False  Point  Lighthouse,  barometric 
pressure  at,  299. 


Famine,  cause  of,  103;  in  England, 
101  f.;  in  India,  104  f.;  pulsations 
theory  and,  87  f. 

Faunas,  and  mild  climates,  168  f . ; 
in  Permian,  152  f . 

Fennoscandian  pause,  216. 

Flowering,  light  and,  184. 

Fog,  and  glaciation,  116;  as  pro- 
tection, 197;  temperature  and, 
178. 

Forests,  climate  and,  66. 

Form  of  the  land,  43  ff. 

Fossil  floras,  and  mild  climates,  168 ; 
in  Antarctica,  273;  in  Greenland, 
273. 

Fossils,  169,  230;  and  loess,  158; 
Archeozoic,  3  f . ;  Cenozoic,  21 ; 
dating  of,  153;  glaciation  and, 
138;  in  peat  bogs,  271;  mild  cli- 
mate, 167;  Proterozoic,  4,  6  f . 

Fourteenth  century,  climatic  stress 
in,  98-109. 

Fowle,  F.  E.,  cited,  45,  237,  238, 
239. 

Freeh,  F.,  cited,  36. 

Free,  E.  E.,  cited,  142. 

Freezing,  salinity  and,  224. 

Fresno,  rainfall  record,  82. 

"Friction  variables,"  247. 

Frisian  Islands,  storm-flood,  99. 

Fritz,  H.,  cited,  109. 

Frogs,  distribution  of,  202. 

Fuchs,  cited,  289. 

Galaxy,  252. 

Galveston,  Tex.,  rainfall  and  tem- 
perature, 94. 

Garner,  W.  W.,  cited,  183,  184. 

Gases,  in  air,  233. 

Geographers,  and  climatic  changes, 
65  ff. 

Geological  time  table,  *  5. 

Geologic  oscillations,  18  f.,  21  ff., 
188,  240. 

Geologists,  changes  in  ideas  of,  64  f . 

Germanic  myths,  219. 

Germany,  forests,  220;  growth  of 
trees  in,  102;  storms  in,  102. 

Gilbert,  G.  K.,  cited,  143. 


INDEX 


Glacial  epochs,  causes  of,  268;  dates 
of,  216;  intervals  between,  264  f.; 
length  of,  166  f. 

Glacial  fluctuations,  24  ff.;  nature 
of,  57  ff. 

Glacial  period,  at  present,  272;  ice 
in,  57  f.;  length  of,  269;  list,  265; 
temperature,  38. 

Glaciation,  and  loess,  155  f.;  and 
movement  of  erust,  287;  condi- 
tions favorable  for,  111;  extent 
of,  124;  hypotheses  of,  33  ff.;  in 
southern  Canada,  18;  localization 
of,  130  ff.;  Permian,  *145;  solar- 
cyclonic  hypothesis  of,  110-129; 
suddenness  of,  138;  upper  limit 
of,  141. 

Goldthwait,  J.  W.,  cited,  271. 

Gondwana  land,  21,  204. 

Gravitation,  effect  on  sun,  250;  pull 
of,  244. 

Great  Basin,  in  glacial  period,  126; 
salt  lakes  in,  142. 

Great  Ice  Age,  see  Pleistocene. 

Great  Plains,  effect  on  ice  sheet,  120. 

Greenland,  climatic  stress,  105  ff . ; 
ice,  26;  rainfall,  108;  storminess, 
57;  submergence,  219;  vegetation, 
21,  37,  287;  winds,  135,  161. 

Gregory,  J.  W.,  cited,  90  ff.,  97. 

Gschnitz  stage,  216. 

Guatemala,  ruins  in,  95. 

Guervain,  cited,  135. 

Gyroscope,  earth  as,  181. 

Hale,  G.  E.,  cited,  56,  62. 

Hamdulla,  cited,  104. 

Hann,  J.,  cited,  66. 

Hansa  Union,  operations  of,  100. 

Harmer,  F.  W.,  cited,  115,  119. 

Heat,  and  earthquakes,  292;  earth's 

internal,  18. 
Hedin,  S.,  cited,  88. 
Heim,  A.,  cited,  190. 
Heligoland,  flood  in,  99. 
Helland-Hansen,  B.,  cited,  174. 
Helmert,  F.  E.,  cited,  302. 
Henderson,  L.   J.,  cited,  9,   10,  11, 

12. 


Henry,  A.  J.,  cited,  94,  208. 

Hercynian  Mountains,  45. 

High  pressure  and   glaciation,   115, 

135. 
Himalayas,    glaciation,    144;    origin 

of,  193;   snow  line,   139. 
Himley,  cited,  104. 
Historic  pulsations,  24  f . ;  nature  of, 

57  ff. 
History,  climate  of,  64-97;  climatic 

pulsations  and,  26. 
Hobbs,  W.  H.,  cited,  115,  125,  135, 

161. 

Hot  springs,  temperature  of,  6. 
Humphreys,   W.   J.,   cited,   2,   37  f., 

45,  46,  50,  56,  238. 
Hurricanes,    in    arid    regions,    144; 

sunspots  and,  53. 
Hyades,  cluster  in,  268. 

Ice,   accumulations,   57f.;    advances 

of,     122;     distribution     of,     131; 

drift,  105. 
Ice     sheets,      disappearance,      128; 

limits,    120 ;    localization,    130  ff . ; 

rate    of    retreat,    165;    thickness, 

125. 

Iceland,  submergence,  219. 
lowan  ice  sheet,  rapid  retreat,  165. 
lowan  loess,  158. 
India,      drought,      104  f . ;      famine, 

104 f.;  rainfall,  108. 
Indian  glaciation,  266. 
Inter-glacial  epoch,  Permian,  153. 
Internal  heat  of  earth,  212. 
Ireland,    Drumkelin    Bog,    220;    in 

glacial  period,  119;  level  of  land, 

220;      storminess     and     rainfall, 

107;  submergence,  219. 
Irish  Sea,  tides,  191. 
Irrigation  ditches,  abandoned,  97. 
Isostasy,  307  ff. 
Italy,  southern,  climate  of,  86  f . 

Japan,  earthquakes  of,  296. 
Javanese  mountains,  origin  of,  193. 
Jaxartes,  108. 

Jeans,  J.   H.,  cited,  251,  252,  253, 
266,  272. 


324 


INDEX 


Jeffreys,  H.,  cited,  302,  303,  306. 

Jeffreys,  J.,  cited,  191. 

Jericho,  palms  in,  92. 

Jerusalem,  rainfall,  86;  rainfall  and 
temperature,  94;  rainfall  in,  and 
sequoia  growth,  83  ff. 

Johnson,  cited,  226. 

Judea,  palms  in,  92. 

Jupiter,  and  sunspots,  243;  effect  of, 
253;  periodicity  of,  61  f.;  tem- 
perature of,  258;  tidal  effect  of, 
250. 

Jurassic,  climate,  266;  mountain 
ranges,  44. 

Kansas,  variations  of  seasons,  103. 

Kara  Koshun  marsh,  Lop  Nor,  104. 

Keewatin  center,  113;  evaporation 
in,  129. 

Keewatin  ice  sheet,  121. 

Kelvin,  Lord,  cited,  13  f . 

Keyes,  C.  B.,  cited,  156. 

Kirk,  E.,  cited,  287. 

Knott,  C.  G.,  cited,  294,  295,  297, 
299,  304,  306. 

Knowlton,  F.  H.,  cited,  167,  169, 
170,  212,  232. 

Koppen,  W.,  47,  52,  140. 

Krakatoa,  glaciation  and,  48;  vol- 
canic hypothesis  and,  45. 

Kriimmel,  O.,  cited,  224,  228. 

Kullmer,  C.  J.,  cited,  113,  115,  128; 
map  of  storminess,  *  54. 

Kungaspegel,  sea  routes  described, 
106. 

Labor,  price  in  England,  102. 

Labradorean  center  of  glaciation, 
113. 

Lahontan,  Lake,  142. 

Lake  strands,  see  Strands. 

Lake  Superior,  lava,  211. 

Lakes,  during  glacial  periods, 
141  f . ;  in  semi-arid  regions,  60 ; 
of  Great  Basin,  126;  ruins  in,  97. 

Land,  and  water,  climatic  effect  of, 
196  ff.;  distribution  of,  200; 
form  of,  43  ff. ;  range  of  tem- 
perature and,  196. 


Lavas,  climatic  effect  of,  211. 

Lawson,  A.  C.,  cited,  310. 

Lebanon,  cedars  of,  83. 

Leiter,  H.,  cited,  71. 

Leverett,  F.,  cited,  271. 

Life,  atmosphere  and,  229  f . ;  chemi- 
cal characteristic  of,  12;  effect  of 
salinity,  225;  of  glacial  period, 
127;  persistence  of  forms,  230. 

Light,  effect  of  atmosphere  on,  236; 
effect  on  plants,  184  ff . ;  ultra- 
violet, storminess  and,  56;  varia- 
tion of,  275. 

Litorina  sea,  218. 

Loess,  date  of,  156  ff . ;  origin  of, 
155,  165. 

Lop  Nor,  rise  of,  104;  swamps,  171. 

Lows,  and  glacial  lobes,  122;  move- 
ments of,  126;  see  Storms  and 
Cyclones. 

Lulan,  104. 

Lull,  E.  S.,  cited,  5,  188. 

MacDougal,  D.  T.,  cited,  171. 

McGee,  W.  J.,  cited,  156. 

Macmillan,  W.  D.,  cited,  191. 

Magdalenian  period,  216. 

Magnetic  fields  of  sunspots,  56. 

Magnetic  poles,  relation  to  storm 
tracks,  150. 

Makran,  climate,  89;  rainfall,  89. 

Malay  Archipelago,  earthquakes  of, 
296. 

Mallet,  E.,  cited,  288. 

Malta,  rainfall,  86. 

Manson,  M.,  cited,  147. 

Mayas,  civilization,  26;  ruins,  95. 

Mayence,  flood  at,  99. 

Mazelle,  E.,  cited,  224. 

Mediterranean,  climate  of,  72;  rain- 
fall records,  86;  storminess  in,  60. 

Mercury,  and  sunspots,  243. 

Mesozoic,  climate,  266;  crustal 
changes,  286;  emergence  of  lands, 
287. 

Messier,  8;  variables,  248. 

Metcalf,  M.  M.,  cited,  202. 

Meteorological  factors  and  earth- 
quakes, 300  f. 


INDEX 


325 


Meteorological  hypothesis  of  crustal 

movements,  294. 

Meteors,  and  sun's  heat,  13,  246. 
Michelson,  A.  A.,  cited,  259. 
Middle    Silurian,    fauna    in    Alaska, 

287. 

Mild  climates,  see  Climates,  mild. 
Milky  Way,  252. 
Mill,  H.  E.,  cited,  228. 
Milne,  J.,  cited,  288,  290,  294,  304, 

306. 

Miocene,  erustal  changes,  287. 
Mississippi  Basin,  loess  in,  159. 
Mogul  emperor,  and  famine,  104. 
Monsoons,  character  of,  146;   direc- 
tion of,  208;  Indian  famines  and, 

105. 

Moulton,  F.  E.,  cited,  13,  258,  269. 
Mountain  building,  climatic  changes 

and,  *  25. 
Mountains,  folding  of,  190;  rainfall, 

on,  208. 
Multiple  stars,  252. 

Nansen,  F.,  cited,  122,  174. 

Naples,  rainfall,  86. 

Nathorst,  cited,  169. 

Nebulae,  247. 

Nebular  hypothesis,  232,  267. 

Neolithic  period,  218. 

Nevada,  correlations  of  rainfall,  86. 

New  England,  height  of  land,  111. 

New  Mexico,  rainfall,  89. 

New  Orleans,  La.,  rainfall  and  tem- 
perature, 94. 

New  Zealand,  climate,  177;  tree 
ferns,  179. 

Newcomb,  S.,  cited,  52. 

Nile  floods,  periodicity  in,  245. 

Nitrogen,  in  atmosphere,  19. 

Niya,  Chinese  Turkestan,  desiccation 
at,  66. 

Nocturnal  cooling,  changes  in,  238  f . 

Norlind,  A.,  cited,  100. 

Norsemen,  route  to  Greenland,  26. 

Norse  sagas,  219. 

North  Africa,  climate  of,  71;  Ro- 
man aqueducts  in,  71. 

North  America,  at  maximum  glacia- 


tion,  122ff.;  emergence  of  lands, 

193;  glaciation  in,  131;  height  of 

land,    111;    interior   sea   in,   200; 

inundations,    196;    loess    in,    155; 

submergence  of  lands,  19,  21. 
North  Atlantic  Ocean,  salinity,  228. 
North    Sea,    climatic    stress,    98  ff.; 

floods    around,    26,    99;    rainfall, 

107;  storminess,  57. 
Northern     hemisphere,     earthquakes 

of,  294. 
Norway,    decay,    100;    temperature, 

177. 
Novse,  247. 

Oceanic  circulation,  carbon  dioxide 
and,  39  ff. 

Oceanic  climate,  characteristics,  103. 

Oceanic  currents,  diversion,  44;  in- 
fluence of  land  distribution,  203. 

Oceans,  age  of,  223;  composition  of, 
223-241;  deepening  of,  199;  sa- 
linity, 19,  223 ;  temperature,  6, 152, 
180,  226. 

Okada,  T.,  cited,  224. 

Old  Testament,  temperature,  92. 

Orbital  precessions,  27. 

Ordovician,  climate,  266. 

Organic  evolution,  glacial  fluctua- 
tions and,  26. 

Orion,  nebulosity  near,  247;  stars 
near,  248. 

Orontes,  67. 

Osborn,  H.  F.,  cited,  216. 

Owens-Searles,  lakes,  142. 

Oxus,  108. 

Oxygen,  in  atmosphere,  20,  234;  in 
Permian,  152. 

Ozone,  cause  of,  56. 

Paleolithic,  216. 

Paleozoic,    climate,    266;    mountains 

in,  209. 

Palestine,  change  of  climate,  91  f . 
Palms,   climatic   change  and,   91  f.; 

in  Ireland,  179. 
Palmyra,  ruins  of,  66. 
Parallaxes  of  stars,  276  f. 
Patrician  center,  134. 


326 


INDEX 


Peat-bog  period,  first,  218. 

Penck,  A.,  cited,  139,  156,  157,  158, 
269. 

Pennsylvania!!,  life  of,  26. 

Periodicities,  245  f. 

Periodicity,  of  climatic  phenomena, 
601;  of  glaciation,  268;  of  sun- 
spots,  243. 

Permian,  climate,  266;  distribution 
of  glaciation,  152;  glaciation,  60, 
144,  *145,  226;  glaciation  and 
mountains,  45;  life  of,  26;  red 
beds,  151 ;  temperature,  146  f. 

Perry,  cited,  289. 

Persia,  lakes,  143;  rainfall,  89. 

Pettersson,  O.,  cited,  98  ff.,  100  f., 
103,  106,  219. 

Pirsson,  L.  V.,  cited,  3,  196. 

Planetary  hypothesis,  253,  267. 

Planetary  nebulae,  252. 

Planets,  and  sunspots,  243;  effect 
of  star  on,  255;  sunspot  cycle 
and,  62 ;  temperatures,  8  f . 

Plants,  climate  and,  1  f . ;  effect  of 
light,  184  ff. 

Pleion,  defined,  29. 

Pleionian  migrations,  29  f . 

Pleistocene,  climate,  266;  duration 
of,  48;  glaciation,  110  ff.;  ice 
sheets,  *  123. 

Pluvial  climate,  causes  of,  143; 
during  glacial  periods,  141. 

Po,  frozen,  98. 

Polaris,  272. 

Polar  wandering,  hypothesis  of,  48  f . 

Pole  and  earthquakes,  305. 

Post-glacial  crustal  movements  and 
climatic  changes,  215-222. 

Poynting,  J.  H.,  cited,  8. 

Precessional  hypothesis,  34  f . 

Precipitation,  and  glaciation,  114, 
133;  during  glacial  period,  118; 
snow  line  and,  139;  temperature 
and,  94. 

Procyon,  companion  of,  280;  lumi- 
nosity, 278;  speed  of,  281. 

Progressive  change,  241. 

Progressive  desiccation,  hypothesis 
of,  65  ff. 


Proterozoic,  4  f . ;  fossils,  6  f . ;  gla- 
ciation, 18,  144,  226,  266;  lava, 
211;  mountains  in,  209;  oceanic 
salinity,  42  f . ;  oxygen  in  air,  234 ; 
red  beds,  151;  temperature,  146  f. 

Pulsations,  hypothesis  of,  65,  72  ff. 

Pulsatory  climatic  changes,  72  ff. 

Pulsatory  hypothesis,  272. 

Pumpelly,  R.,  cited,  271. 


Eadiation,  variation  of,  275. 

Eadioactivity,  heat  of  sun  and,  14  f . 

Rainfall,  changes  in,  93  f . ;  glacia- 
tion and,  50;  sunspots  and,  53, 
*58,  59;  tree  growth  and,  79. 

Red  beds,  151,  170. 

Rhine,  flood,  99;  frozen,  98. 

Rho  Ophiuchi,  variables,  248. 

"Rice  grains,"  61. 

Richardson,  O.  W.,  cited,  256. 

Rigidity,  of  earth,  307. 

Roads,  climate  and,  66. 

Rogers,  Thorwald,  cited,  101. 

Romans,  aqueduct  of,  71. 

Rome,  history  of,  87. 

Rotation,  of  earth,  18  f. 

Ruden,  storm-flood,  99. 

Rugen,  storm-flood,  99. 

Ruins,  as  climatic  evidence,  66 ;  rain- 
fall and,  60. 

Sacramento,  correlation  coefficients, 
82  f.,  85;  rainfall,  86;  rainfall 
record,  79. 

Sagas,  cited,  105  f. 

St.  John,  C.  E.,  cited,  236. 

Salinity,  deep-sea  circulation  and, 
176;  effect  on  climate,  224;  in 
North  Atlantic,  228;  ocean  tem- 
perature and,  226;  of  ocean,  19, 
120. 

Salisbury,  R.  D.,  cited,  111,  125,  129, 
139,  156,  206,  269,  271. 

Salt,  in  ocean,  223. 

San  Bernardino,  correlation  of  rain- 
fall, 85. 

Saturn,  and  sunspots,  243;  sunspot 
cycle  and,  62. 


INDEX 


327 


Sayles,  E.  W.,  cited,  183. 

Scandinavia,  climatic  stress,  100  f . ; 
fossils,  271;  post-glacial  climate, 
271;  rainfall,  107;  storminess,  57, 
107;  temperature,  216. 

Scandinavian  center  of  glaciation, 
113. 

Schlesinger,  P.,  cited,  275,  278,  298, 
301,  305. 

Schuchert,  C.,  cited,  3,  5,  23,  *  25, 
*  123,  138,  *  145,  168,  169,  172, 
188,  193,  196,  198,  200,  *  201,  206, 
211,  230,  265. 

Schuster,  A.,  cited,  61,  244,  294,  296. 

Sculpture,  Maya,  96. 

Sea  level  and  glaciation,  119. 

Seasonal  alternations,  28  f . 

Seasonal  banding,  183  f . 

Seasonal  changes,  geological,  183. 

Seasons,  and  earthquakes,  294,  295, 
297,  299;  evidences  of,  169. 

Secular  progression,  17  ff.,  188. 

Seistan,  swamps,  171. 

Sequoias,  measurements  of,  74  ff . ; 
rainfall  record,  79. 

Setchell,  W.  A.,  cited,  1. 

Shackleton,  E.,  cited,  125. 

Shapley,  H.,  cited,  246,  247,  254, 
256,  275. 

Shimek,  E.,  cited,  157,  161. 

Shreveport,  La.,  rainfall  and  tem- 
perature, 93  f. 

Shrinkage  of  the  earth,  190. 

Siberia,  and  glaciation,  132. 

Sierras,  rainfall  records,  82. 

Simpson,  G.  C.,  cited,  222. 

Sirius,  companion  of,  280;  distance 
from  sun,  262;  luminosity,  278; 
speed  of,  281. 

Slichter,  C.  S.,  cited,  192. 

Smith,  J.  W.,  cited,  73. 

Snowfall,  glaciation  and,  50,  114. 

Snowfield,  climatic  effects  of,  115. 

Snow  line,  height  of,  138;  in  Andes, 
139;  in  Himalayas,  139. 

Solar  activity,  cycles  of,  245;  deep- 
sea  circulation  and,  179;  ice  and, 
134. 

Solar  constant,  114. 


Solar-cyclonic  hypothesis,  51-63, 
287;  glaciation  and,  110-129. 

Solar  prominences,  cause  of,  61. 

Solar  system,  252;  conservation  of, 
243;  proximity  to  stars,  63. 

Solar  variations,  storms  and,  31. 

South  America,  earthquakes,  301. 

South  Pole,  thickness  of  ice  at,  125. 

Southern  hemisphere,  earthquakes, 
296;  glaciation  in,  131  f. 

Southern  Pacific  railroad,  rainfall 
records  along,  82. 

Soy  beans,  effect  of  light,  185  f. 

Space,  sun's  journey  through,  264- 
284. 

Spiral  nebulae,  25 If.;  universe  of, 
267. 

Spitsbergen,  submergence,  219. 

Springs,  climate  and,  66. 

Stars,  approach  to  sun,  253;  binary, 
252;  clusters,  252,  268;  effect  on 
solar  atmosphere,  63;  dark,  254; 
parallaxes  of,  276  f . ;  tidal  action 
of,  249. 

Stefan 's  Law,  257. 

Stein,  M.  A.,  cited,  78. 

Stellar  approaches,  probability  of, 
260. 

Storm  belt  in  arid  regions,  144. 

Storm-floods,  in  fourteenth  century, 
99. 

Storminess,  and  erosion,  309;  and 
ice,  134;  effect  on  glaciation,  112; 
sun  spots  and,  163;  temperature 
and,  94,  173. 

Storms,  blows  of,  300,  302;  increase, 
60;  movement  of,  125f.;  move- 
ment of  water  and,  *  175 ;  origin 
of,  3 Of.;  sunspots  and,  28,  53; 
see  Cyclones  and  Lows. 

Storm  tracks,  during  glacial  period, 
117;  location,  113;  relation  to 
magnetic  poles,  150;  shifting  of, 
119. 

Strands,  climate  and,  66;  in  semi- 
arid  regions,  60 ;  of  salt  lakes,  142. 

Suess,  E.,  cited,  192. 

Sun,  and  the  earth's  crust,  285-317; 
approach  to  star,  253;  atmosphere 


328 


INDEX 


of,  61,  274;  atmosphere  of,  and 
weather,  52;  cooling  of,  49;  con- 
traction of,  249;  disturbances  of, 
172 ;  effect  of  other  bodies  on,  242- 
263;  heat,  13;  journey  through 
space,  264-284;  Knowlton's  hy- 
pothesis of,  168. 

Suncracks,  232. 

Sunspot  cycles,  27  f. 

Sunspots,  and  earthquakes,  289; 
causes  of,  61;  magnetic  field  of, 
261;  maximum  of,  109;  mild  cli- 
mates and,  172;  number,  108f.; 
periodicity,  243;  planetary  hy- 
pothesis of,  253;  records,  245; 
storminess  and,  163;  storms  and, 
300;  temperature  of  earth  and, 
52,  173. 

Sunspot  variations,  282. 

Swamps,  as  desert  phenomena,  171. 

Sylt,  storm-flood,  99. 

Syria,  civilization  in,  67;  inscrip- 
tions in,  76;  Eoman  aqueducts  in, 
71. 

Syrian  Desert,  ruins  in,  66. 

Talbert,  cited,  213. 

Tarim  Basin,  red  beds,  151. 

Tarim  Desert,  desiccation,  66. 

Tarim  Eiver,  swamps,  171. 

Taylor,  G.,  cited,  140,  144,  191,  271. 

Temperature,  change  of  in  Atlantic, 
174;  changes  in,  93;  climatic 
change  and,  49;  critical,  9;  geo- 
logical time  and,  3 ;  glacial  period, 
38;  glaciation  and,  42,  132,  139; 
gradient  of  earth,  213;  of  ocean, 
180;  in  Norway,  177;  in  Permian, 
146  f . ;  in  Proterozoic,  146  f . ; 
limits,  6  ff . ;  precipitation  and,  94 ; 
range  of,  3,  8;  solar  activity  and, 
140;  storminess  and,  94,  112,  173; 
sunspots  and,  28,  173;  volcanic 
eruptions  and,  46;  zones,  172. 

Terrestial  causes  of  climatic  changes, 
188-214. 

Tertiary,  lava,  211. 

Thames,  frozen,  98. 

Thermal  solar  hypothesis,  49  f .,  97. 


Thermo-pleion,   movements   of,  30. 

Thesis,  of  pulsations,  24. 

Thiryu,  storm-flood,  99. 

Tian-Shan  Mountains,  irrigation  in, 
71. 

Tidal  action  of  stars,  249. 

Tidal  effect,  of  Jupiter,  253;  of 
planets,  244. 

Tidal  hypothesis,  251. 

Tidal  retardation,  effect  on  land  and 
sea,  191;  rotation  of  earth  and, 
18  f.;  stress  caused  by,  310. 

Tides,  cycles  of,  219. 

Time,  geological,  see  Geological 
time. 

Toads,  distribution  of,  202. 

Tobacco  plant,  effect  of  light,  184. 

Topography,  and  glaciation,  132. 

Transcaspian  Basin,  red  beds,  151. 

Tree  ferns,  in  New  Zealand,  179. 

Tree  growth,  periodicity  in,  245; 
rainfall  and,  79. 

Trees,  in  California,  219;  measure- 
ment of,  73  ff. 

Triassic,  climate,  266. 

Trifid  Nebula,  variables,  248. 

Trondheim,  wheat  in,  101. 

Trondhenas,  corn  in,  101. 

Tropical  cyclones,  in  glacial  epochs, 
140f.;  occurrence,  148;  solar  ac- 
tivity and,  113. 

Tropical  hurricanes,  earthquakes 
and,  300;  sunspots  and,  149. 

Turf  an,  temperature,  17. 

Turner,  H.  H.,  cited,  245. 

Tyler,  J.  M.,  cited,  216. 

Tyndall,  J.,  cited,  36,  37. 

Typhoon  region,  ' '  earthquake 
weather,"  298. 

Typhoons,  occurrence,  300. 

United  States,  rainfall  and  tempera- 
ture in  Gulf  region,  93  f . ;  salt 
lakes  in,  142;  southwestern,  cli- 
mate, 66 ;  storminess,  53  f .,  60. 

Variables,  247. 
Veeder,  M.  A.,  cited,  300. 
Vegetation,  theory  of  pulsations  and, 
90. 


INDEX 


Venus,  atmosphere  of,  236. 

Vesterbygd,  invasion  of,  106. 

Vicksburg,  Miss.,  rainfall  and  tem- 
perature, 93  f . 

Volcanic  activity,  climate  and,  210; 
movement  of  the  earth's  crust 
and,  285;  times  of  uplifting  lands 
and,  23. 

Volcanic  dust,  climatic  changes  and, 
97. 

Volcanic  hypothesis,  climatic  change 
and,  45  ff . ;  snow  line,  139. 

Volcanoes,  activity  of,  96. 

Volga,  108. 

Walcott,  C.  D.,  cited,  4,  230. 

Wandering  of  the  pole,  302. 

Water,  importance,  9. 

Water  vapor,  condensation  of,  56; 
effect  on  life,  231;  in  atmos- 
phere, 19. 

Wave,  effect  on  movement  of  water, 
176. 

Weather,  changes  of,  3 If.;  origin 
of,  174;  variations,  52. 

Wells,  H.  G.,  cited,  35. 


Wendingstadt,  storm-flood,  99. 

Westerlies,  21  f. 

Wheat,  price  in  England,  102. 

White  Sea,  submergence,  219. 

Whitney,  J.  D.,  cited,  142. 

Wieland,  G.  R.,  cited,  169. 

Williamson,  E.  D.,  cited,  226. 

Willis,  B.,  cited,  206. 

Winds,  at  ice  front,  162;  effect  on 

currents,  174;  glaciation  and,  133; 

in    Antarctica,     161;     in    glacial 

period,    119;    in   Greenland,    161; 

planetary  system  of,  174 ;  velocity, 

240. 

Witch  hazel,  effect  of  light,  184. 
Wolf,  J.  E.,  cited,  61,  109,  288. 
Wolfer,  cited,  244. 
Wright,  W.  B.,  cited,  35,  111,  119. 
Writing,  among  Mayas,  96. 

Yucatan,  Maya  civilization,  26,  107; 

rainfall,  108;  ruins,  95. 
Yukon,  Ice  Age  in,  221. 

Zante,  earthquakes  of,  296. 
Zonal  crowding,  117. 


PRINTED  IN  THE  UNITED  STATES  OF  AMERICA 


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cop.  2 


H-untington  - 


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